Crossword Genius

Something bird has put by? (4,3)

Ross

I believe the answer is:

nest egg

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'put by?' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

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

Can you help me to learn more?

(Other definitions for nest egg that I've seen before include "Money laid aside for some future eventuality" , "Sum of money saved in avian manner" , "Rainy day savings" , "Sum of money saved, as bird would" , "Savings for the future" .)

I've seen this clue in The Mirror.
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