You should probably write a script to do this
but assuming you have just user tar (or somthing
like that) to get the file on the tape, upon
successful completion, you could simply do:

> some_file.log

...this will effectively truncate some_file.log
I suggest a shell so that you can check the
exit code for the tar command then based on
success or failure, the script could then
choose to truncate the file or perhaps send
you an email notifcation of the problem.