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' indicates anagramming the letters.
'repents' is an anagram of '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" .)