Zwieback (4)
I believe the answer is:
rusk
'zwieback' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for rusk that I've seen before include "Baby's dry biscuit" , "It's hard for babe to get teeth into" , "Bread baked again until it is brown and hard" , "Baby's teething biscuit" , "cake" .)