Gun has removed third rule (5)
I believe the answer is:
canon
'rule' is the definition.
(canon can mean a law)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for canon that I've seen before include "religious leader" , "General rule; clergyman" , "Senior clergyman" , "Literary works of quality" , "Standard" .)