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' - private school pupil).
'round' becomes 'o' (o is a circular letter).
'with' means one lot of letters go next to another.
'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" .)