A not half sad affair (5)
I believe the answer is:
amour
'affair' is the definition.
('amour' can be a synonym of 'affair')
'a not half sad' is the wordplay.
'not half' suggests halving the letters.
'sad' becomes 'mournful' (I've seen this before).
'mournful' cut in half is 'mour'.
'a'+'mour'='AMOUR'
(Other definitions for amour that I've seen before include "close relationship" , "Love affair - loved one" , "Loved one (French)" , "A secret love affair in France" , "Romantic liaison" .)