Antacid (6)
I believe the answer is:
bicarb
'antacid' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for bicarb that I've seen before include "What helps raise" , "source of fizz?" , "baking aid" , "Raising agent in baking (colloq.)" , "It helps digestion" .)