Rule head teacher read aloud (9)
I believe the answer is:
principle
'rule' is the definition.
(I know that rule can be written as principle)
'head teacher read aloud' is the wordplay.
'head teacher' becomes 'principal' (synonyms).
'read aloud' shows a homophone (sound like).
'principal' sounds like 'PRINCIPLE'.
(Other definitions for principle that I've seen before include "Law of nature, rule of conduct" , "honour" , "Set of rules" , "Fundamental truth - moral rule" , "Basic standard" .)