Spoil drop of Courvoisier brandy (4)
I believe the answer is:
marc
'brandy' is the definition.
(marc is a kind of brandy)
'spoil drop of courvoisier' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'drop of' says to take the initial letters (the smallest possible amount of the word).
The first letter of 'courvoisier' is 'c'.
'mar'+'c'='MARC'
(Other definitions for marc that I've seen before include "Chagall for one" , "Cram (anag.)" , "Sort of brandy made from the remains of pressed grapes" , "Brandy distilled from grape residue" , "Fruit refuse" .)