Possesses heroin at first and pot (4)
I believe the answer is:
hash
'pot' is the definition.
(hash is a kind of pot)
'possesses heroin at first' is the wordplay.
'possesses' becomes 'has' ('have' can be a synonym of 'possess').
'at first' indicates taking the first letters.
The initial letter of 'heroin' is 'h'.
'has'+'h'='HASH'
'and' is the link.
(Other definitions for hash that I've seen before include "Mess; meat/potato dish" , "chop into small pieces" , "What signifies a number" , "Recooked dish for shah" , "Keypad symbol" .)