It's an illegal substance - the man's in a mess (7)
I believe the answer is:
hashish
'it's an illegal substance' is the definition.
I can't tell whether this definition defines the answer.
'man's in a mess' is the wordplay.
'man's' becomes 'His' (belonging to a man).
'in' indicates putting letters inside.
'a mess' becomes 'hash' (I've seen this before).
'his' placed into 'hash' is 'HASHISH'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for hashish that I've seen before include "Cannabis" , "Pot" , "Hallucinogenic extract of hemp" , "Resinous product of hemp smoked as drug" , "Grass" .)