Not acid (6)
I believe the answer is:
alkali
'not acid' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for alkali that I've seen before include "Opposite of acid" , "Basic stuff" , "Substance such as lime or caustic soda" , "Soluble compound that turns litmus paper blue" , "In pH terms, not acid" .)