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' means one lot of letters go next to another.
'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" .)