Possesses heroin and marijuana (4)
I believe the answer is:
hash
'marijuana' is the definition.
(hash is a kind of marijuana)
'possesses heroin' is the wordplay.
'possesses' becomes 'has' ('have' can be a synonym of 'possess').
'heroin' becomes 'h' ('H' can be a synonym of 'heroin').
'has'+'h'='HASH'
'and' acts as a link.
(Other definitions for hash that I've seen before include "chop into small pieces" , "Meat dish or hallucinogen" , "Dish of diced meat etc - cannabis (slang)" , "Chopped meat mixed with potatoes and browned" , "A finely chopped mixture of meat and potatoes" .)