Upright, not upper class, old expert musician (8)
I believe the answer is:
virtuoso
'expert musician' is the definition.
I don't understand how the remainder of the clue works.
(Other definitions for virtuoso that I've seen before include "Musician with exceptional technique" , "Showing dazzling skill" , "Highly skilled musician" , "Person who excels in techniques of singing or playing music" , "maestro?" .)