Guiding principle (5)
'guiding principle' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tenet that I've seen before include "A belief or credo - it's a palindrome" , "Fundamental belief" , "Doctrine, dogma, belief" , "A belief or creed - a palindrome" , "Opinion held to be true" .)
(Other definitions for ethic that I've seen before include "Code of principles" , "Ideology" , "Set of moral principles" , "set of morals" , "System of rules of behaviour" .)
(Other definitions for precept that I've seen before include "Rule of conduct or doctrine" , "Moral rule" , "order for payment of rate" , "Canon" , "Rubric" .)