Bitter enemy (7)
I believe the answer is:
nemesis
(Other definitions for nemesis that I've seen before include "causes downfall" , "rival who can't be beaten" , "Nee Miss (anag.)" , "Divinity" , "opponent you can't beat" .)
I believe the answer is:
nemesis
(Other definitions for nemesis that I've seen before include "causes downfall" , "rival who can't be beaten" , "Nee Miss (anag.)" , "Divinity" , "opponent you can't beat" .)