Cocktail I had as a youngster (3)
I believe the answer is:
kid
'a youngster' is the definition.
(synonyms)
'cocktail i had' is the wordplay.
'cocktail' becomes 'K' (the end letter or tail of the word 'cocktail').
'i had' becomes 'i'd' (contraction).
'k'+'id'='KID'
'as' acts as a link.
(Other definitions for kid that I've seen before include "Suede" , "Schoolboy" , "To fool; young animal" , "Pretend" , "Josh" .)