Opponent's for change, not against (10)
I believe the answer is:
antagonist
'opponent's' is the definition.
(synonyms)
'change not against' is the wordplay.
'change' indicates anagramming the letters.
'not'+'against'='notagainst'
'notagainst' anagrammed gives 'ANTAGONIST'.
'for' is the link.
(Other definitions for antagonist that I've seen before include "Adversary" , "Opponent" , "rival" , "Hostile person" , "5 down (anag)" .)