Former pupil round with eastern instrument (4)
I believe the answer is:
oboe
'instrument' is the definition.
(type of musical instrument)
'former pupil round with eastern' is the wordplay.
'former pupil' becomes 'OB' (abbreviation for 'Old Boy').
'round' becomes 'o' (round can mean a round object).
'with' says to put letters next to each other.
'eastern' becomes 'e'.
'ob'+'o'+'e'='OBOE'
(Other definitions for oboe that I've seen before include "Walk with exaggerated movements" , "Double-reed wind instrument" , "it's blown" , "Double-reed woodwind" , "issuer of notes" .)