Former wife put on pounds, sweetheart's loss (7)
I believe the answer is:
expense
'loss' is the definition.
(expense is a kind of loss)
'former wife put on pounds sweetheart's' is the wordplay.
'former wife' becomes 'ex'.
'put on' says to put letters next to each other.
'pounds' becomes 'pens'.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'ex'+'pens'+'e'='EXPENSE'
(Other definitions for expense that I've seen before include "What something costs" , "Disbursement" , "Outlay or cost" , "Incurred cost" , "damage" .)