Tart-tasting (6)
I believe the answer is:
acidic
I believe this clue is a double definition.
'tart' is the first definition.
(I've seen this before)
'tasting' is the second definition.
(I have seen 'Sharp tasting' mean 'acidic' so perhaps 'tasting' could also mean 'acidic')
(Other definitions for acidic that I've seen before include "Like vinegar" , "Sarcastic or sour" , "Tart, sour" , "Tasting sharp or sour" , "Sour; bitter" .)