Head teacher's rule read aloud (9)
I believe the answer is:
principal
'head teacher's' is the definition.
(synonyms)
'rule read aloud' is the wordplay.
'rule' becomes 'principle' (synonyms).
'read aloud' indicates a 'sounds like' (homophone) clue.
'principle' is a homophone of 'PRINCIPAL'.
(Other definitions for principal that I've seen before include "Capital invested" , "Headmaster" , "Primary" , "property" , "Stop" .)