Swap old coins (8)
I believe the answer is:
exchange
'swap' is the definition.
(I've seen this before)
'old coins' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'coins' becomes 'change' (change can mean coins).
'ex'+'change'='EXCHANGE'
(Other definitions for exchange that I've seen before include "Stock trading site" , "Substitute" , "Bandy" , "Altercation" , "Barter; switch" .)