Demand former lover behave (5)
I believe the answer is:
exact
'demand' is the definition.
(exacting is a kind of demanding)
'former lover behave' is the wordplay.
'former lover' becomes 'ex'.
'behave' becomes 'act' (acting is a kind of behaving).
'ex'+'act'='EXACT'
(Other definitions for exact that I've seen before include "Precise and strictly correct" , "dead" , "Force out" , "Precise, accurate" , "Not approximate, precise" .)