Eve's tempter, when reformed, repents (7)
I believe the answer is:
serpent
'eve's tempter' is the definition.
(I've seen this before)
'reformed repents' is the wordplay.
'reformed' is an anagram indicator.
'repents' anagrammed gives 'SERPENT'.
'when' is the link.
(Other definitions for serpent that I've seen before include "Present (anag.)" , "Present for a snake" , "Devil of a snake" , "Eve's tempter" , "Tempting" .)