I'm using FCP HD.
How can I correct dialog recorded via a lav that was clipped (too loud) when it was recorded?

The dialog can be easily understood, but when the actor speaks too loud it becomes distorted. Can anything be done in post to clean this up, make it less distorted?

TIA.

I'm pretty sure that there isn't much you can do about that. Try lowering your volume in the parts that clip but it is still going to sound bad.

try passing it though a low pass filter as that will remove all the high frequency components of the signal and remove the sharp edges where it is clipped. Your milage may vary with this as you'll need to pick a cutoff frequency that doesn't also distort the voice of the speaker by removing the frequency components of his/her voice.

Let us know how you get on...

You should put the mic in auto in future if you cannot watch the levels all the time. This happens to me sometimes as well so I'm not slagging you. Best to err on the lower gain if doing it manually to handle these peaks. you can always lift the levels later in post.
Donie