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