Baby's biscuit (4)
I believe the answer is:
rusk
'baby's biscuit' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rusk that I've seen before include "Capital of Malta" , "Bread baked again until it is brown and hard" , "Hard bread given to babies" , "in Bristol Channel" , "Baby's dry biscuit" .)