The morning belonging to us for an affair (5)
I believe the answer is:
amour
'an affair' is the definition.
('amour' can be a synonym of 'affair')
'the morning belonging to us' is the wordplay.
'the morning' becomes 'am'.
'belonging to us' becomes 'our' (I've seen this before).
'am'+'our'='AMOUR'
'for' is the link.
(Other definitions for amour that I've seen before include "Result of Cupid's darts" , "Romantic affair" , "Love affair, perhaps secret" , "A secret love affair in France" , "20 [AFFAIR]" .)