Opponent, adversary (10)
I believe the answer is:
antagonist
I believe this clue is a double definition.
'opponent' is the first definition.
(synonyms)
'adversary' is the second definition.
(synonyms)
(Other definitions for antagonist that I've seen before include "Adversary" , "Hostile person" , "rival" , "Opponent" , "5 down (anag)" .)