Sharp or tart (4)
I believe the answer is:
acid
I believe this is a double definition.
'sharp' is the first definition.
(as in sharp-tasting)
'tart' is the second definition.
(both can mean bitter-tasting)
(Other definitions for acid that I've seen before include "Polluted precipitation" , "Sour - sharp" , "Sour to the taste" , "See 4 Across" , "Type of chemical" .)