Adversary not against being messed around (10)
I believe the answer is:
antagonist
'adversary' is the definition.
(synonyms)
'not against being messed around' is the wordplay.
'being messed around' indicates an anagram (I've seen 'messing around' mean this).
'not'+'against'='notagainst'
'notagainst' with letters rearranged gives 'ANTAGONIST'.
(Other definitions for antagonist that I've seen before include "Opponent" , "rival" , "Adversary" , "Hostile person" , "One who is hostile" .)