Regret drinking shot served up for villain (5)
I believe the answer is:
rogue
'for villain' is the definition.
(rogue is a kind of villain)
'regret drinking shot served up' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'drinking' indicates putting letters inside (inserted letters are consumed or drunk).
'shot' becomes 'go' (both can mean an attempt).
'served up' says the letters should be written backwards (in a down clue, going upwards).
'go' written backwards gives 'og'.
'rue' placed around 'og' is 'ROGUE'.
(Other definitions for rogue that I've seen before include "Baddie" , "Fraud" , "Scoundrel, knave" , "Wild animal driven away from herd" , "Stray" .)