Sweet's counterpart (4)
I believe the answer is:
sour
'sweet's counterpart' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sour that I've seen before include "Bitter, tart" , "Tasting sharp like stale milk" , "Sharp- or tart-tasting" , "Acid or caustic" , "Tart, acidic" .)