Standard, the rule (4)
I believe the answer is:
norm
This clue is a double definition.
'standard' is the first definition.
(I know that norm is a type of standard)
'the rule' is the second definition.
(norm is a kind of rule)
(Other definitions for norm that I've seen before include "Something typical" , "Standard amount or customary behaviour" , "Typical, usual" , "Standard choice" , "Standard considered to be representative" .)