A drink needed by first of children to be expelled (4,3)
I believe the answer is:
cast out
'be expelled' is the definition.
(I've seen this before)
'a drink needed by first of children' is the wordplay.
'drink' becomes 'stout' (type of beer).
'needed by' is a charade indicator (letters next to each other).
'first of children' becomes 'c' (1st letter of 'children').
'a'+'stout'='astout'
'astout' after 'c' is 'CAST OUT'.
'to' acts as a link.
(Other definitions for cast out that I've seen before include "Expel" , "Expelled" , "Rejected from a group" .)