Crossword Genius

Cuckoo — bird seen before end of May (5)

Ross

I believe the answer is:

loony

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'cuckoo' is the definition.
(thesaurus)

'bird seen before end of may' is the wordplay.
'bird' becomes 'loon' (loon is a kind of bird).
'seen before end of may' becomes 'y' (I can't explain this - if you can you should give a lot more credence to this answer).
'loon'+'y'='LOONY'

Can you help me to learn more?

(Other definitions for loony that I've seen before include "Crazy" , "Nuts" , "mad" , "Silly" .)

I've seen this clue in The Times.
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