A tango isn't performed for opponent (10)
I believe the answer is:
antagonist
'for opponent' is the definition.
(synonyms)
'a tango isn't performed' is the wordplay.
'performed' indicates an anagram.
'tango'+'isnt'='tangoisnt'
'tangoisnt' anagrammed gives 'ntagonist'.
'a'+'ntagonist'='ANTAGONIST'
(Other definitions for antagonist that I've seen before include "Opponent" , "rival" , "Hostile person" , "Adversary" , "5 down (anag)" .)