They often have a flutter (5)

I believe the answer is:

birds

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'they' is the definition.
Both the definition and answer are plural nouns.
Maybe you can see an association between them that I don't see?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for birds that I've seen before include "4th" , "Feathered fliers" , "Egg-laying vertebrates" , "Terns and tits, say" , "Gulls" .)

I've seen this clue in the Evening Standard.

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