Trade in old coins, say (8)
I believe the answer is:
exchange
'trade in' is the definition.
(I've seen this before)
'old coins say' is the wordplay.
I cannot really understand how this works, but
'old' could be 'ex' (prefix meaning former or onetime) and 'ex' is found within the answer.
'coins' could be 'change' (change can mean coins) and 'change' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for exchange that I've seen before include "Swap, barter" , "market" , "truck" , "place of business" , "Price of one currency in another" .)