In favour of compromise (3)
I believe the answer is:
pro
'in favour' is the definition.
(I've seen this before)
'of compromise' is the wordplay.
'of' indicates the answer is hidden within the clue.
'PRO' is hidden in the letters of 'compromise'.
(Other definitions for pro that I've seen before include "Expert (abbr)" , "Golfer" , "In favour of; advantage" , "Favouring the professional" , "In brief, he's no amateur" .)