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' (I've seen this in another clue).
'ex'+'change'='EXCHANGE'
(Other definitions for exchange that I've seen before include "Stock trading site" , "Substitute" , "Bandy" , "Altercation" , "Barter; switch" .)