Archrival (7)
I believe the answer is:
nemesis
'archrival' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nemesis that I've seen before include "Greek goddess - opponent who cannot be beaten" , "An agent of vengeance and retribution" , "Bane" , "Figure of retribution" , "E'en miss (anag.)" .)