A bet by a bird? (7)

I believe the answer is:

flutter

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'a bet' is the definition.
(I have seen 'Small bet' mean 'flutter' so perhaps 'bet' could also mean 'flutter')

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for flutter that I've seen before include "Informally a slight gamble as on a horse" , "Bat" , "Fly unsteadily; small bet" , "Small bet; beat feebly" , "Move one's eyelashes - move like a butterfly - bet" .)

I've seen this clue in the Evening Standard.

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