Is in possession of the chap’s hard drug (7)
I believe the answer is:
hashish
'drug' is the definition.
(hashish is a kind of drug)
'is in possession of the chap's hard' is the wordplay.
'in possession of' indicates putting letters inside.
'the chap' becomes 'hash' (I can't justify this - if you can you should give a lot more credence to this answer).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'hash'+'h'='hashh'
'is' going within 'hashh' is 'HASHISH'.
Can you help me to learn more?
(Other definitions for hashish that I've seen before include "Hallucinogenic extract of hemp" , "Pot" , "Dope" , "Hemp hallucinogen" , "Cannabis" .)