>I have a very old application written in Clipper. Now as per the need I wish to make changes in it. I have opened it in Valkirye, a software that could give me the code of the application .
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>However, now I want to make some changes in it and again compile it in clipper itself. Any Idea , how to compile the same in clipper again.

Perhaps you can download clipper compiler somewhere from web and then just re-compile code into exe again.
I believe Clipper should work w/o problems in DOS mode. Perhaps some people here can dig up and send you usable clipper tool set. Mine got buried for good on those 5.25 floppies (right here in my desk) and I doubt I can recover anything from there.

Another (IMO better) option would be to use FPD 2.6 to create executables. Apart from having to convert data to fox format (not that dificult) changes to code should be minimal.

Good luck.