Drug in dish is heroin (7)
I believe the answer is:
hashish
'drug' is the definition.
(hashish is a kind of drug)
'dish is heroin' is the wordplay.
'dish' becomes 'hash' (hash is a kind of dish).
'heroin' becomes 'h' ('H' can be a synonym of 'heroin').
'hash'+'is'+'h'='HASHISH'
'in' acts as a link.
(Other definitions for hashish that I've seen before include "Cannabis" , "Pot" , "Dope" , "Resinous product of hemp smoked as drug" , "Hemp hallucinogen" .)