Do you have any idea what vintage the clipper code is? My first guess would be 5.0, and second would be Summer '87, but there are many other options. Depending on which version was used to compile the EXE, there are different tools that you can use to retreive the source code.
Another "interesting" feature that might cause you trouble is the use of various purchased libraries. Those can significantly complicate issues for you.
Unfortuantely I have no idea which vintage Clipper it is. It was created long before my time at the company, by contractors, and by now the paperwork is unfortuantely pretty sketchy. I will try to find out, but do you have any ideas which tools I could start with in the meantime?
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.