__ cracker (4)
I believe the answer is:
soda
'cracker' is the definition.
(soda cracker is a kind of cracker)
This is all the clue.
(Other definitions for soda that I've seen before include "Type of bread, water or fountain" , "Mixer drink" , "Sodium carbonate" , "Alkaline substance used in cleaning, drinks etc" , "In the US, a sweet fizzy drink" .)