You won't find this in the middle of yearbooks the four masters produced (6)

I believe the answer is:

annals

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'you won't find this in the middle of yearbooks the four masters produced' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

This is all the clue.

Can you help me to learn more?

(Other definitions for annals that I've seen before include "Historical records" , "Yearly records" , "History of events year by year" , "Registers" , "Archives" .)

I've seen this clue in the Irish Times and the Irish Times (Crosaire).

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