Make youngster's first drink (6)
I believe the answer is:
brandy
'drink' is the definition.
(brandy is a kind of drink)
'make youngster's first' is the wordplay.
'make' becomes 'brand' (synonyms).
'youngster's first' becomes 'y' (1st letter of 'youngster').
'brand'+'y'='BRANDY'
(Other definitions for brandy that I've seen before include "See 20" , "Spirit from grapes" , "Strong alcoholic spirit" , "Grape wine spirit" , "Cognac, calvados, armagnac etc" .)