Opponent's not against spinning (10)
I believe the answer is:
antagonist
'opponent's' is the definition.
('antagonist' can be a synonym of 'opponent')
'not against spinning' is the wordplay.
'spinning' indicates an anagram.
'not'+'against'='notagainst'
'notagainst' is an anagram of 'ANTAGONIST'.
(Other definitions for antagonist that I've seen before include "Adversary" , "Hostile person" , "5 down (anag)" , "Opponent" , "One who is hostile" .)