Note cocaine and smack (4)
I believe the answer is:
chit
'note' is the definition.
(I've seen this before)
'cocaine and smack' is the wordplay.
'cocaine' becomes 'C' (drugs abbreviation).
'and' means one lot of letters go next to another.
'smack' becomes 'hit' (I've seen this in another clue).
'c'+'hit'='CHIT'
(Other definitions for chit that I've seen before include "Brief note" , "Docket; pert girl" , "Informal receipt" , "Pert young lady" , "Impudent girl" .)