Wife to go down after hot date – that's a bonus (8)
I believe the answer is:
windfall
'that's a bonus' is the definition.
(windfall is a kind of bonus)
'wife to go down after hot date' is the wordplay.
'wife' becomes 'w' (abbreviation).
'to go down' becomes 'fall' (falling is a kind of going down).
'after' says to put letters next to each other.
'hot' becomes 'in' (both can mean fashionable).
'date' becomes 'd' (abbreviation e.g. dob - date of birth).
'in'+'d'='ind'
'fall' after 'ind' is 'indfall'.
'w'+'indfall'='WINDFALL'
(Other definitions for windfall that I've seen before include "Stroke of luck" , "Unexpected bonus" , "unexpected blessing" , "Godsend" , "Fruit that has fallen from the tree or sudden good fortune" .)