Multiplies notes on liqueur (6,3)
I believe the answer is:
triple sec
'liqueur' is the definition.
(triple sec is a kind of liqueur)
'multiplies notes on' is the wordplay.
'multiplies' becomes 'triples' (tripling is a kind of multiplying).
'notes' becomes 'ec'.
'on' means one lot of letters go next to another.
'triples'+'ec'='TRIPLE SEC'
(Other definitions for triple sec that I've seen before include "Drink" , "Strong orange-flavoured liqueur - perils etc (anag)" , "alcohol" .)