Officer, in short, not wrong about one sharing authority (9)
I believe the answer is:
collegial
'one sharing authority' is the definition.
(describing shared responsibility or authority)
'officer in short not wrong about' is the wordplay.
'officer' becomes 'col' (short for colonel).
'short' means to remove the last letter.
'not wrong' becomes 'legal'.
'about' is an insertion indicator.
'in' with its final letter taken away is 'i'.
'i' going into 'legal' is 'legial'.
'col'+'legial'='COLLEGIAL'
(Another definition for collegial that I've seen is " Pertaining to, say, Eton".)