They return bags left during parties (9)

I believe the answer is:

carousels

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'they' is the definition.
Both the definition and answer are plural nouns.
Perhaps they are linked in a way I don't understand?

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

Can you help me to learn more?

(Another definition for carousels that I've seen is " Roundabouts".)

I've seen this clue in The Independent.

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