Former wife serves demands (6)
I believe the answer is:
exacts
'demands' is the definition.
(exacting is a kind of demanding)
'former wife serves' is the wordplay.
'former wife' becomes 'ex'.
'serves' becomes 'acts' (acting is a kind of serving).
'ex'+'acts'='EXACTS'
(Other definitions for exacts that I've seen before include "making demands" , "Compels to furnish" , "Insists on and obtains - sound precise" , "Extorts" , "Insists on and obtains, say payment - sounds precise" .)