Old boy with old English instrument (4)
I believe the answer is:
oboe
'instrument' is the definition.
(type of musical instrument)
'old boy with old english' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'with' means one lot of letters go next to another.
'old english' becomes 'OE'.
'ob'+'oe'='OBOE'
(Other definitions for oboe that I've seen before include "Double-reed wind instrument" , "music producer" , "Double-reed woodwind" , "Player" , "Source of music" .)