Rule to live by (5)
I believe the answer is:
tenet
'rule to live by' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tenet that I've seen before include "Belief - maxim" , "That's the law" , "Doctrine, dogma, belief" , "Dogma or principle" , "Item of belief" .)