Kept woman in standard affair (8)
I believe the answer is:
Here is my best explanation:
'kept woman' is the definition.
(paramour is a kind of kept woman)
'standard affair' is the wordplay.
'standard' becomes 'par' (I've seen this before).
'affair' becomes 'amour' ('amour' can be a synonym of 'affair').
'in' acts as a link.