Swap
'swap' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for exchange that I've seen before include "Barter; switch" , "Substitute" , "Convert" , "Stock trading site" , "Price of one currency in another" .)
'swap' is the definition.
(I know that swap can be written as trade)
This is all the clue.
(Other definitions for trade that I've seen before include "Deal commercially" , "Buy and sell in business" , "groundsman?" , "Buy and sell, barter" , "Transaction" .)
(Other definitions for replace that I've seen before include "Substitute, supplant" , "Provide a substitute for" , "succeed" , "Put something back where it belongs" , "Put back or substitute" .)