Forestall ex-pupil by way of a note (7)
I believe the answer is:
obviate
'forestall' is the definition.
(I've seen this before)
'ex-pupil by way of a note' is the wordplay.
'ex pupil' becomes 'ob' (abbreviation for 'Old Boy').
'by way of' becomes 'via' (I've seen this before).
'a note' becomes 'te'.
'ob'+'via'+'te'='OBVIATE'
(Other definitions for obviate that I've seen before include "Preclude" , "Render unnecessary" , "Prevent from happening" , "Make unnecessary" , "Get round inconvenience - get above it" .)