Acidic (4)
'acidic' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tart that I've seen before include "fruit in this dish" , "quiche, perhaps" , "Professional woman" , "Pie with fruit filling" , "Of taste, sharp or acid" .)
'acidic' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for sour that I've seen before include "Acid-tasting" , "Acid tasting" , "Acetic" , "'Having a sharp, biting taste (4)'" , "Curdled" .)