Former partner and communist returning to force (6)
I believe the answer is:
extort
'to force' is the definition.
(thesaurus)
'former partner and communist returning' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'and' is a charade indicator (letters next to each other).
'communist' becomes 'trot' (short for Trotskyist).
'returning' says the letters should be written in reverse.
'trot' reversed gives 'tort'.
'ex'+'tort'='EXTORT'
(Other definitions for extort that I've seen before include "Squeeze" , "Obtain (money etc) by force" , "To bleed" , "Gain using compulsion" , "Obtain money with menaces" .)