Demanding former partner performing (8)
I believe the answer is:
exacting
'demanding' is the definition.
('exact' can be a synonym of 'demand')
'former partner performing' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'performing' becomes 'acting' (acting is a kind of performing).
'ex'+'acting'='EXACTING'
(Other definitions for exacting that I've seen before include "Requiring great effort - sounds precise" , "Requiring hard work and attention" , "Testing endurance or skill" , "Arduous and demanding" , "Insisting upon" .)