Compelled former pupil to glide off (7)
I believe the answer is:
obliged
'compelled' is the definition.
('oblige' can be a synonym of 'compel')
'former pupil to glide off' is the wordplay.
'former pupil' becomes 'OB' (abbreviation for 'Old Boy').
'off' indicates an anagram.
'glide' is an anagram of 'liged'.
'ob'+'liged'='OBLIGED'
(Other definitions for obliged that I've seen before include "Constrained" , "Did favour to, or coerced" , "Made bound (to do something)" , "Bound by a favour rendered" , "Grateful - compelled" .)