Opponent not against change (10)
I believe the answer is:
antagonist
'opponent' is the definition.
(synonyms)
'not against change' is the wordplay.
'change' indicates anagramming the letters.
'not'+'against'='notagainst'
'notagainst' is an anagram of 'ANTAGONIST'.
(Other definitions for antagonist that I've seen before include "5 down (anag)" , "rival" , "Opponent" , "Hostile person" , "One who is hostile" .)