Not sad (5)
I believe the answer is:
happy
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not sad</span>' is the definition.<br/ ><span class="explanation-format__explanation">('happy' is the opposite to 'sad')</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for happy that I've seen before include "Joyous" , "Gleeful" , "Pleased, content" , "Pleased; fortunate" , "Well-pleased" .)