So assuming its a stereo file -
On the pull down near the pan control "Split Stereo Track"
From there you can do it a few ways...
You could just delete the empty track - and from the pull down call the good track "Mono" - and render...
Or you might click in the good track, click the purple back arrow and then Edit > Select Cursor To End > COPY
Repeat above in the bad track - but PASTE. On the pull down call one left and the other right - then Make Stereo Track and render...
Should do it...