Cocktail I had as a youngster (3)
I believe the answer is:
kid
'a youngster' is the definition.
('kid' can be a synonym of 'youngster')
'cocktail i had' is the wordplay.
'cocktail' becomes 'k' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'i had' becomes 'id' (I'd).
'k'+'id'='KID'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for kid that I've seen before include "Suede" , "Schoolboy" , "To fool; young animal" , "Pretend" , "Josh" .)