Cardinal rule, reportedly (9)
I believe the answer is:
principal
'cardinal' is the definition.
(I've seen this before)
'rule reportedly' is the wordplay.
'rule' becomes 'principle' (principle is a kind of rule).
'reportedly' indicates a 'sounds like' (homophone) clue.
'principle' sounds like 'PRINCIPAL'.
(Other definitions for principal that I've seen before include "British civil servant" , "Chief - teacher?" , "Cardinal" , "Stop" , "Primary" .)