Substitute for an old flame with cash (8)
I believe the answer is:
exchange
'substitute' is the definition.
(exchanging is a kind of substituting)
'an old flame with cash' is the wordplay.
'an old flame' becomes 'ex' (I've seen this in other clues).
'with' is a charade indicator (letters next to each other).
'cash' becomes 'change' (change is a kind of cash**).
'ex'+'change'='EXCHANGE'
'for' is the link.
(Other definitions for exchange that I've seen before include "place of business" , "Give one thing and get another back" , "Altercation" , "Barter; switch" , "Price of one currency in another" .)