Wife struck Frenchman over fake cream in coffee (8)
I believe the answer is:
whitener
'fake cream in coffee' is the definition.
(substitute for cream)
'wife struck frenchman over' is the wordplay.
'wife' becomes 'w'.
'struck' becomes 'hit'.
'frenchman' becomes 'rene' (common French name).
'over' shows that the letters should be reversed in order.
'rene' backwards is 'ener'.
'w'+'hit'+'ener'='WHITENER'
(Other definitions for whitener that I've seen before include "Powdered coffee additive" , "Blanco" .)