Former contributor to school discipline (3)
I believe the answer is:
old
'former' is the definition.
(both can mean previous)
'contributor to school discipline' is the wordplay.
'contributor to' indicates a hidden word (the letters of the answer contribute to the clue).
'OLD' can be found hidden inside 'school discipline'.
(Other definitions for old that I've seen before include "Long-lived" , "Ancient" , "SECOND PART OF 14" , "Venerable" , "Senescent" .)