At the end of the day, have reformed a villain (5)
I believe the answer is:
heavy
'a villain' is the definition.
(I've seen this before)
'at the end of the day have reformed' is the wordplay.
'at the end of' indicates one should take the final letters.
'reformed' indicates an anagram.
The final letter of 'day' is 'y'.
'y'+'have'='yhave'
'yhave' is an anagram of 'HEAVY'.
(Other definitions for heavy that I've seen before include "Villain" , "Stodgy; difficult" , "Of great weight" , "serious!" , "Not light" .)