Heard headmaster's belief (9)
I believe the answer is:
principle
'belief' is the definition.
(thesaurus)
'heard headmaster's' is the wordplay.
'heard' indicates a 'sounds like' (homophone) clue.
'headmaster' becomes 'principal' (I've seen this before).
'principal' sounds like 'PRINCIPLE'.
(Other definitions for principle that I've seen before include "Fundamental truth - moral rule" , "Basic standard" , "Axiom" , "Basic truth, law or assumption" , "Morally correct behaviour" .)