Expert, virtuoso (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)
'virtuoso' is the second definition.
(I've seen this before)
(Other definitions for master that I've seen before include "With 1a [OLD], one who painted" , "Gain control of (something)" , "get control over" , "Virtuoso" , "Teacher to gain control" .)