Bill supporting former lover's demand (5)
I believe the answer is:
exact
'demand' is the definition.
(exacting is a kind of demanding)
'bill supporting former lover's' is the wordplay.
'bill' becomes 'act' (I've seen this in other clues).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'former lover' becomes 'ex'.
'act' after 'ex' is 'EXACT'.
(Other definitions for exact that I've seen before include "Completely accurate" , "Absolutely right" , "dead" , "Call" , "Specific" .)