Possessing heroin - after comedian... (4)
I believe the answer is:
with
'possessing' is the definition.
(I've seen this before)
'heroin after comedian' is the wordplay.
'heroin' becomes 'H' (H is a slang term for heroin).
'after' is a charade indicator (letters next to each other).
'comedian' becomes 'wit' (a witty person).
'h' after 'wit' is 'WITH'.
(Other definitions for with that I've seen before include "In the presence of" , "Accompanied by" , "Among" , "In spite of" , "Alongside" .)