Youngster's little drink (3)
I believe the answer is:
tot
'drink' is the definition.
(I've seen this before)
'youngster's little' is the wordplay.
'youngster's' becomes 'tots' (tot is a kind of youngster).
'little' means to remove the last letter (the word is not quite complete).
'tots' with its last letter taken away is 'TOT'.
(Other definitions for tot that I've seen before include "Get the sum for the child" , "Add; small child" , "Small measure for a small child" , "Small amount of spirits" , "Child's" .)