Futile incarcerating bad, bad guy (7)
I believe the answer is:
villain
'bad guy' is the definition.
(I know that bad guy can be written as villain)
'futile incarcerating bad' is the wordplay.
'futile' becomes 'vain' (synonyms).
'incarcerating' indicates putting letters inside (I've seen this in other clues).
'bad' becomes 'ill' (associated in meaning).
'vain' placed around 'ill' is 'VILLAIN'.
(Other definitions for villain that I've seen before include "Wicked criminal" , "Evil character" , "Principal fictional baddie" , "Criminal, rogue" , "Wrongdoer" .)