Least happy (7)
I believe the answer is:
saddest
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">least happy</span>' is the definition.<br/ ><span class="explanation-format__explanation">('sad' is the opposite of 'happy' so 'saddest' should be a synonym for 'least happy')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for saddest that I've seen before include "most dejected" , "Most doleful" , "Unhappiest" , "Most unhappy" , "Most blue" .)