Exchange prisoners of war the other way (4)

I believe the answer is:

swop

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'exchange' is the definition.
(I know that swop is a more specific form of the action exchange)

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

Can you help me to learn more?

(Another definition for swop that I've seen is " Exchange (Alt sp.)".)

I've seen this clue in The Irish News.

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