Motorway force kept woman (8)
I believe the answer is:
mistress
'kept woman' is the definition.
('mistress' can be a synonym of 'kept woman')
'motorway force' is the wordplay.
'motorway' becomes 'mi' (resembles Ms, English motorway).
'force' becomes 'stress' (stress is a kind of force**).
'mi'+'stress'='MISTRESS'
(Other definitions for mistress that I've seen before include "she wears the trousers?" , "Fancy woman" , "Lady" , "Schoolteacher" , "Female owner of a dog" .)