Villain's regret accepting turn about (5)
I believe the answer is:
rogue
'villain's' is the definition.
(rogue is a kind of villain)
'regret accepting turn about' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'accepting' is an insertion indicator.
'turn' becomes 'go' (a person's move in a board game**).
'about' is a reversal indicator.
'go' reversed gives 'og'.
'rue' enclosing 'og' is 'ROGUE'.
(Other definitions for rogue that I've seen before include "Chap in gallery" , "Dishonest man" , "Diseased plant" , "Bad guy" , "Prankster" .)