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It may pop out for an hour (6)

Ross

I believe the answer is:

cuckoo

I'm a little stuck... Click here to teach me more about this clue!

This could be a double definition.

'it may pop out' is the first definition.
The definition suggests a singular noun which matches the answer. I also know that 'pop out' relates to this answer.

'an hour' is the second definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'for' acts as a link.

Can you help me to learn more?

(Other definitions for cuckoo that I've seen before include "One calling on the hour" , "Bird which lays eggs in anothers nest" , "Unwelcome bird in the nest" , "Hourly you might hear this" , "Bats" .)

I've seen this clue in The Globe and Mail.
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