Left the bad guy out for being nasty (4)
I believe the answer is:
ugly
'nasty' is the definition.
(I've seen this in another clue)
'left the bad guy out' is the wordplay.
'left' becomes 'l' (common abbreviation).
'the bad' indicates anagramming the letters.
'out' means one lot of letters goes inside another.
'guy' anagrammed gives 'ugy'.
'l' inserted into 'ugy' is 'UGLY'.
'for being' acts as a link.
(Other definitions for ugly that I've seen before include "Deficient in beauty" , "Not nice-looking" , "Disquieting" , "Not nice to look at" , "Monstrous" .)