Behave after former lover's demand (5)
I believe the answer is:
exact
'demand' is the definition.
(I know that exact is a type of demand)
'behave after former lover's' is the wordplay.
'behave' becomes 'act' (synonyms).
'after' is a charade indicator (letters next to each other).
'former lover' becomes 'ex' (an ex-lover).
'act' put after 'ex' is 'EXACT'.
(Other definitions for exact that I've seen before include "Correct; insist on" , "Not approximate; inflict (revenge) on someone" , "Spot-on" , "Explicit" , "As a right, demand" .)