Each little gun is exchanged for old gold coins (7)

I believe the answer is:

guineas

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'coins' is the definition.
(guinea is a kind of coin)

I don't understand how the rest of the clue works.

(Other definitions for guineas that I've seen before include "Former gold coins" , "They used to be worth 21 shillings" , "Old money - run at Newmarket" , "Obsolete coins (still used at Newmarket?)" .)

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

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