Precept, rule (9)
I believe the answer is:
principle
I believe this clue is a double definition.
'precept' is the first definition.
(I know that precept can be written as principle)
'rule' is the second definition.
(I know that principle is a type of rule)
(Other definitions for principle that I've seen before include "Set of rules" , "Basic standard" , "Rule of personal conduct" , "Basic truth, law or assumption" , "Fundamental truth - moral rule" .)