Why the songbird couldn't sleep? (11)

I believe the answer is:

nightingale

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'why' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for nightingale that I've seen before include "The Lady with the Lamp, d. 1910" , "Keats's immortal Bird" , "Bird - famous nurse" , "Singer appearing every season" , "Migratory songbird" .)

I've seen this clue in The Irish News.

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