Barter old shrapnel (8)

I believe the answer is:

exchange

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'barter' is the definition.
(I've seen this before)

'old shrapnel' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'shrapnel' becomes 'change' (I can't explain this - if you can you should believe this answer much more).
'ex'+'change'='EXCHANGE'

Can you help me to learn more?

(Other definitions for exchange that I've seen before include "Stock trading site" , "place of business" , "Convert" , "Altercation" , "market" .)

I've seen this clue in the Sydney Morning Herald.

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