A friend initially upstaging ringleader, not professional (7)
I believe the answer is:
amateur
'not professional' is the definition.
('amateur' is the opposite to 'professional')
'a friend initially upstaging ringleader' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend).
'initially' indicates taking the first letters.
The initial letters of 'upstaging ringleader' is 'ur'.
'a'+'mate'+'ur'='AMATEUR'
(Other definitions for amateur that I've seen before include "Unpaid (sportsman}" , "Unprofessional" , "A non-professional sportsman" , "Unpaid practitioner" , "Person who does something as a pastime rather than a profession" .)