Crossword Genius

Unconvincing sort of reasoning by judges in official bulletin (5,8)

Ross

I believe the answer is:

court circular

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'bulletin' is the definition.
Both the definition and answer are singular nouns.
Maybe there's an association between them I don't understand?

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for court circular that I've seen before include "report of royals" , "Report of royal activities" , "list of engagements" .)

I've seen this clue in The Times.
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