Former pupil with old European instrument (4)
I believe the answer is:
oboe
'instrument' is the definition.
(type of musical instrument)
'former pupil with old european' is the wordplay.
'former pupil' becomes 'OB' (abbreviation for 'Old Boy').
'with' says to put letters next to each other.
'old' becomes 'o' (abbreviation).
'european' becomes 'E' (abbreviation e.g. EU).
'ob'+'o'+'e'='OBOE'
(Other definitions for oboe that I've seen before include "Musical instrument" , "Reed instrument" , "Producer of notes" , "it's blown" , "bit of wind" .)