Sour, tart (6)
I believe the answer is:
acidic
I believe this is a double definition.
'sour' is the first definition.
(similar in meaning)
'tart' is the second definition.
(I've seen this in another clue)
(Other definitions for acidic that I've seen before include "Vinegary" , "Sour; bitter" , "Sarcastic or sour" , "Containing a sour-tasting substance" , "that leaves a bitter taste" .)