Former wife finishing with Bill? Spot on (5)
I believe the answer is:
exact
'spot on' is the definition.
(spot on means exactly right)
'former wife finishing with bill?' is the wordplay.
'former wife' becomes 'ex' (an ex-wife).
'finishing with' means one lot of letters go next to another.
'bill?' becomes 'act' (I've seen this in other clues).
'ex'+'act'='EXACT'
(Other definitions for exact that I've seen before include "Demand, extort" , "Precise - demand" , "Demand and obtain" , "on the money" , "Finished" .)