Acid neutralizer (6)
I believe the answer is:
alkali
'acid neutralizer' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for alkali that I've seen before include "Salt" , "Non-acid" , "Soluble compound that turns litmus paper blue" , "Basic stuff" , "Substance such as lime or caustic soda" .)