Kept woman by force on motorway (8)
I believe the answer is:
mistress
'kept woman' is the definition.
('mistress' can be a synonym of 'kept woman')
'force on motorway' is the wordplay.
'force' becomes 'stress' (stress is a kind of force**).
'on' is a charade indicator (letters next to each other).
'motorway' becomes 'mi' (resembles Ms, English motorway).
'stress' after 'mi' is 'MISTRESS'.
'by' is the link.
(Other definitions for mistress that I've seen before include "Kept woman" , "lover" , "Fancy woman" , "Lady" , "Woman teacher" .)