Impromptu session in the morning after joint (3)
I believe the answer is:
jam
'impromptu session' is the definition.
(impromptu musical session)
'morning after joint' is the wordplay.
'morning' becomes 'am' (as in 12:00 AM).
'after' means one lot of letters go next to another.
'joint' becomes 'J' (drug term).
'am' put after 'j' is 'JAM'.
'in the' acts as a link.
(Other definitions for jam that I've seen before include "awful traffic" , "Sweet stuff for bread - stuff it in" , "Spread; improvise (jazz)" , "Troublesome situation" , "Block signals" .)