Least sweet (6)
I believe the answer is:
driest
'least sweet' is the definition.
('dry' is the opposite of 'sweet' so 'driest' should be a synonym for 'least sweet')
This is the entire clue.
(Other definitions for driest that I've seen before include "with least rainfall" , "Most arid - take it in stride" , "Least moist" .)