Sour sweet? (4)
I believe the answer is:
tart
I believe this clue is a double definition.
'sour' is the first definition.
(similar in meaning)
'sweet?' is the second definition.
(I've seen this before)
(Other definitions for tart that I've seen before include "Open pastry filled with apple or jam" , "Sharp, acidic" , "Tom" , "Tasting sour like a lemon" , "Flat pie" .)