Established rule (5)
I believe the answer is:
axiom
'established rule' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for axiom that I've seen before include "Plainly true proposition" , "Accepted statement taken as self-evidently true" , "Accepted general truth or principle" , "It's taken for granted" , "Fundamental proposition" .)