An early caller (7)

I believe the answer is:

rooster

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'an early caller' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

This is all the clue.

Can you help me to learn more?

(Other definitions for rooster that I've seen before include "Farmyard bird" , "Morning caller" , "One crows" , "Farmyard cock" , "Male domestic fowl (US)" .)

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

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