Chief reported truth (9)
I believe the answer is:
principal
'chief' is the definition.
(synonyms)
'reported truth' is the wordplay.
'reported' indicates a 'sounds like' (homophone) clue.
'truth' becomes 'principle' (I have seen 'Fundamental truth ' mean 'principle' so perhaps 'truth' could also mean 'principle').
'principle' is a homophone of 'PRINCIPAL'.
(Other definitions for principal that I've seen before include "British civil servant" , "Chief - teacher?" , "property" , "Key" , "Cardinal" .)