Broad sash initially introduced by former pupil (3)
I believe the answer is:
obi
'broad sash' is the definition.
(I've seen this before)
'initially introduced by former pupil' is the wordplay.
'introduced' indicates taking the first letters.
'by' is a charade indicator (letters next to each other).
'former pupil' becomes 'OB' (abbreviation for 'Old Boy' - private school pupil).
The initial letter of 'initially' is 'i'.
'i' put after 'ob' is 'OBI'.
(Other definitions for obi that I've seen before include "Broad Japanese sash" , "Material band" , "exotic waistband" , "Garment" , "Spells" .)