Expert, genius (6)
I believe the answer is:
master
I believe this clue is a double definition.
'expert' is the first definition.
(I know that master is a type of authority)
'genius' is the second definition.
(thesaurus)
(Other definitions for master that I've seen before include "Lord of a stream" , "Craftsperson of great skill" , "Lord - controller - teacher" , "Male teacher or ship's captain" , "Strong chess player" .)