Joint in a man's jacket (6)
I believe the answer is:
reefer
This clue could be a double definition.
'joint' is the first definition.
(informal term for a marijuana joint)
'man's jacket' is the second definition.
'reefer' can be an answer for 'jacket' (I've seen this before). I'm not sure about the 'man's' bit.
'in a' acts as a link.
Can you help me to learn more?
(Other definitions for reefer that I've seen before include "packed with cannabis?" , "Warm jacket - cannabis cigarette" , "roll of hemp" , "courses may be reduced by one" , "Unofficial smoking" .)