Least happy (7)
I believe the answer is:
saddest
'least happy' is the definition.
('sad' is the opposite of 'happy' so 'saddest' should be a synonym for 'least happy')
This is all the clue.
(Other definitions for saddest that I've seen before include "most dejected" , "Most doleful" , "Unhappiest" , "Most unhappy" , "Most blue" .)