Alcoholic drink's trademark unknown (6)
I believe the answer is:
brandy
'alcoholic drink's' is the definition.
(I've seen this before)
'trademark unknown' is the wordplay.
'trademark' becomes 'brand' (I've seen this before).
'unknown' becomes 'y' (algebra).
'brand'+'y'='BRANDY'
(Other definitions for brandy that I've seen before include "Liquor distilled from wine" , "Strong alcohol" , "Cognac, perhaps" , "eg Armagnac" , "Utensil or shop" .)