Musician turning supersonic first (13)
I believe the answer is:
percussionist
'musician' is the definition.
(percussionist is a kind of musician)
'turning supersonic first' is the wordplay.
'turning' indicates anagramming the letters.
'first' becomes 'ist' (ist resembles 1st).
'supersonic' is an anagram of 'percussion'.
'percussion'+'ist'='PERCUSSIONIST'
(Other definitions for percussionist that I've seen before include "Striking musician" , "Orchestral member" , "Banger" .)