Former wife wrong to blackmail (6)
I believe the answer is:
extort
'blackmail' is the definition.
(thesaurus)
'former wife wrong' is the wordplay.
'former wife' becomes 'ex'.
'wrong' becomes 'tort' (legal term).
'ex'+'tort'='EXTORT'
'to' acts as a link.
(Other definitions for extort that I've seen before include "Blackmail" , "fleece" , "Obtain (money etc) by force" , "Get by intimidation" , "Force to fork out" .)