Rule head teacher's announced (9)
I believe the answer is:
principle
'rule' is the definition.
(principle is a kind of rule)
'head teacher's announced' is the wordplay.
'head teacher' becomes 'principal' ('principal' can be a synonym of 'head teacher').
'announced' shows a homophone (sound like).
'principal' is a homophone of 'PRINCIPLE'.
(Other definitions for principle that I've seen before include "Law of nature, rule of conduct" , "Fundamental truth - moral rule" , "Basic standard" , "Axiom" , "Rule of personal conduct" .)