Not alkaline (6)
I believe the answer is:
acidic
'not alkaline' is the definition.
('acidic' is the opposite to 'alkaline')
This is the entire clue.
(Other definitions for acidic that I've seen before include "Containing a sour-tasting substance" , "Tasting sharp or sour" , "that leaves a bitter taste" , "Like vinegar" , "Cutting" .)