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 a hidden word.
'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" .)