Small amount of drink for youngster (3)
I believe the answer is:
tot
I believe this is a triple definition!
'small amount' is the first definition.
(I've seen this in another clue)
'of' acts as a link.
'drink' is the second definition.
(a small alcoholic drink)
'for youngster' is the third definition.
(tot is a kind of youngster)
(Other definitions for tot that I've seen before include "Small baby" , "Add the little child?" , "Drop" , "Small drink of spirits" , "Get the sum for the child" .)