The dope got his in a mess (7)
I believe the answer is:
hashish
'the dope' is the definition.
(hashish is a kind of dope)
'his in a mess' is the wordplay.
'in' means one lot of letters goes inside another.
'a mess' becomes 'hash' (making a hash of something is making a mess of it).
'his' going inside 'hash' is 'HASHISH'.
'got' is the link.
(Other definitions for hashish that I've seen before include "Resinous product of hemp smoked as drug" , "Hemp hallucinogen" , "Pot" , "Dope" , "Hallucinogenic extract of hemp" .)