Unbeatable rival (7)
I believe the answer is:
nemesis
'unbeatable rival' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nemesis that I've seen before include "Siemens (anag.)" , "Something causing misery (or death) - Meissen (anag)" , "Nee Miss (anag.)" , "Greek goddess - opponent who cannot be beaten" , "causes downfall" .)