Gun has removed third rule (5)

I believe the answer is:

canon

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'rule' is the definition.
(canon can mean a law)

I don't understand the remainder of the clue.

(Other definitions for canon that I've seen before include "religious leader" , "General rule; clergyman" , "Senior clergyman" , "Literary works of quality" , "Standard" .)

I've seen this clue in The Daily Express.

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