Crossword Genius

Tiny shrill sound perhaps ... Young birds (not large) in trees (7,7)

Ross

I believe the answer is:

weeping willows

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'tiny shrill sound perhaps young birds' is the definition.
Although both the answer and definition are plural nouns, I don't understand how they can define each other.

'not large in trees' is the wordplay.
'not large in' becomes 'weeping' (I can't justify this - if you can you should give a lot more credence to this answer).
'trees' becomes 'willows' (willow is a kind of tree).
'weeping'+'willows'='WEEPING WILLOWS'

Can you help me to learn more?

I've seen this clue in The Everyman puzzle in The Observer.
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