Wrongdoer renouncing evil — that's pointless (4)
I believe the answer is:
vain
'pointless' is the definition.
(I've seen this in another clue)
'wrongdoer renouncing evil' is the wordplay.
'wrongdoer' becomes 'villain' (a villain does bad things).
'renouncing' suggests deleting specific letters.
'evil' becomes 'ill' (I've seen this in another clue).
'villain' with 'ill' taken away is 'VAIN'.
'that's' is the link.
(Other definitions for vain that I've seen before include "Empty" , "Producing no result" , "Conceited - or a waste of time" , "Conceited and ineffectual" , "Conceited or unavailing" .)