Sash former pupil originally introduced (3)
I believe the answer is:
obi
'sash' is the definition.
(type of Japanese sash or belt)
'former pupil originally introduced' is the wordplay.
'former pupil' becomes 'OB' (abbreviation for 'Old Boy' - private school pupil).
'originally' suggests taking the first letters.
The first letter of 'introduced' is 'i'.
'ob'+'i'='OBI'
(Other definitions for obi that I've seen before include "Martial arts sash" , "Garment" , "Japanese band" , "magic" , "Broad Japanese sash" .)