Kept woman in standard affair (8)

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').
'par'+'amour'='PARAMOUR'

'in' acts as a link.