Old boy is to supply musician (6)
I believe the answer is:
oboist
'musician' is the definition.
(oboist is a kind of musician)
'old boy is to supply' is the wordplay.
'old boy' becomes 'ob'.
'supply' indicates anagramming the letters (supply of letters to make).
'is'+'to'='isto'
'isto' anagrammed gives 'oist'.
'ob'+'oist'='OBOIST'
(Other definitions for oboist that I've seen before include "Player of woodwind instrument of treble pitch" , "Do I boost the musician?" , "Wind player" , "orchestra member" , "Instrumentalist" .)