Agent of retribution (7)
'agent of retribution' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nemesis that I've seen before include "The Greek goddess of divine retribution and vengeance" , "Vengeful justice" , "Retribution personified" , "curse" , "opponent you can't beat" .)
(Other definitions for avenger that I've seen before include "One exacting retribution" , "One seeking retribution" , "One who exacts reparation for a wrong" , "Retaliator" , "One exacts retribution" .)