In serious adversity, not happy (3)
I believe the answer is:
sad
'not happy' is the definition.
('sad' is the opposite to 'happy')
'in serious adversity' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SAD' is hidden in the letters of 'serious adversity'.
(Other definitions for sad that I've seen before include "Regrettable" , "Unfortunate" , "Mournful - sorry" , "Miserable" , "Dismal" .)