All swans? Not really (5)

I believe the answer is:

geese

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'all swans?' is the definition.
The answer and definition can be both animals as well as being plural nouns.
Perhaps there's an association between them I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for geese that I've seen before include "Stupid people" , "Farmyard creatures" , "Birds; silly people" , "Christmas birds" , "They're seen in a gaggle" .)

I've seen this clue in the Evening Standard.

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