Ex-student has an old English instrument (4)
I believe the answer is:
oboe
'instrument' is the definition.
(type of musical instrument)
'ex-student has an old english' is the wordplay.
'ex-student' becomes 'OB' (abbreviation for 'Old Boy').
'has' is a charade indicator (letters next to each other).
'an old english' becomes 'OE'.
'ob'+'oe'='OBOE'
(Other definitions for oboe that I've seen before include "Producer of notes" , "blow it!" , "Walk with exaggerated movements" , "Source of music" , "Instrument used in 1d" .)