Swap farthings and threepenny bits? (8)
I believe the answer is:
exchange
'swap' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for exchange that I've seen before include "Convert" , "Altercation" , "Price of one currency in another" , "Substitute" , "Give one thing and get another back" .)