Hi David,

You didn't say which version of ACL you are using. I will assume 2013.

There is help in F1 and by clicking on the ? in the declipping tool. See below.

I presume that you read this? I agree that it does not help much and it is not obvious if it does anything. I would expect the wave form to be modified on-screen, but nothing seems to happen. However, it actually does de-clipping. You can try an exaggerated effect on a small clip and then export it to a new wave file and listen to the result, or a reasonable test, export the result, re-import it and observe the change to the waveform.

Before I give the steps, there is a similar tool in Video Sound Cleaning Lab - first version (not 2014) that actually shows the reduced wave form. This appears to be destructive, whereas in ACL it is a real-time non-destructive effect. Thus in ACL it does not change the waveform. But, you will see the result in the exported file upon re-importing it.

Here are the steps:

1. If you have a large file and only part is overmodulated, then you may want to split the section out to only apply the effect to the affected area. However, if you do, you will have to bring the sound back up using the compressor to match the adjacent areas.
2. If you have split the track into more than one object, select the affected object and then click on the Object button at the lower left of the screen (beside Project).
3. Click on the Cleaning button at the lower left. You will now see the Cleaning tools and you will be in Object mode.
4. Click on the On button of the DeClipper and you will see a help screen to the left. If it does not appear, then click on the arrow on the menu line just above the DeClipper tool and to the very right. Read what it says to maybe understand a bit better. It is not all that clear, unfortunately.
5. Select DeClipper toopbox by clicking on the gear button on the tool, or by going to the top menu, Effects, Clean, De-Clipper, same result. You will see the toolbox as per the image that I showed above in this post.
6. Make sure that your playback cursor is in your object and then click on the Get button in the DeClipper. See the help screen as I indicated (or in the F1 Help) to understand what this does.
7. In my example, I exaggerated the Clip level just to better illustrate the level that shows up on the wave form. See next image. You should see a similar but likely smaller range in your test.
8. 
9. You can click on the preset dropdown to the right of the Get button and try them out to see if they help. For me, the Get button returned a Clip level of -0.1 but I changed it to -0.2. Increasing the knob (decreasing the numbers) gave distortion.
10. Once you have selected a Clip level, press on the spacebar to playback the audio. Listen to the result. It may be very difficult to discern any noticeable change.
11. Increase the value using the knob and listen again to what happens, then select a value that gives no distortion, either the Get value or something close.
12. Look at the Output level. As far as I can understand, this is where you will increase gain using the compressor. Try different levels and watch the meters in the toolbox and the upper right of the arranger screen beside the master volume. You will see that the value shows up above this meter. You want the level as high as possible without going into the red.
13. Once you have selected the clip and output levels, listen to what is going to get removed by clicking on the INVERSE  button in the meter screen to the right of the DeClipper toolbax. It is not obvious that this is a button, but it is.
14. Listen to the playback and you will hear what will be removed. To better understand, increase the clip level and play again. When you no longer or barely hear the speech or singing or sound that you want to keep, then you have the correct Clip level.
15. Check again the Output level.
16. When you are satisfied, click on OK. Nothing will happen to the wave form image, but the tool will be active. Check to make sure that the DeClipper is on (it will be highlighted in cyan).
17. Normally, one would continue with the cleaning and mastering, but since this is test of de-clipping only, click on the export button, select WAV and other file formats. Note the recommendations in the help screen to not combine de-clipping with other effects.
18. Type in a filename, note or change the location (at the top of the image where it says Save in), and Export.
19. Save your work in ACL if not already done.
20. Now start a new project in ACL - File, New project.
21. Import the file that you just exported. Look at the wave form and compare it to what you had before. You should see a noticeable difference.
22. See the image below for the result of my test with the Clip level at -0.2.
23. 
24. To better illustrate, here is the before and after image. Note that the waveform is no longer truncated at the top.
25. 

 To see a good example using a different product, see the following tutorial:

I am by no means an expert at this, I just wanted to understand how it works to be able to correct some of my bad recordings. If someone else reading this has suggestions and corrections to the above, please contribure.

David, please let us know how you make out.

To complement the above, I mentioned VSCL making the changes within the program. Here are the before and after screens showing this. ACL does not do this the same way.