If you don't know what version of Clipper was used to create the executable, it would be very time consuming and expensive to simply buy one decompiler after another and try them all. There isn't 100% certainty that it would even succeed, so I can't recommend it.
My first suggestion would be to see if there is a copy of the source code somewhere at your company. Odds are good that a reputable contractor would have provided the source, and somebody in your company should have archived a copy of it somewhere.
If that fails, then find someone that is a local Clipper expert. For a few hundred dollars, they can probably get your source code (or at least most of it) back for you.
-PatP