Demand former partner do something (5)
I believe the answer is:
exact
'demand' is the definition.
(I know that exact is a more specific form of demand)
'former partner do something' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'do something' becomes 'act' (to act is to do something).
'ex'+'act'='EXACT'
(Other definitions for exact that I've seen before include "Spot on, precise" , "totally accurate" , "far from improvised?" , "Strict" , "Precisely accurate" .)