Former pupil's remedy is esoteric (7)
I believe the answer is:
obscure
'esoteric' is the definition.
(thesaurus)
'former pupil's remedy' is the wordplay.
'former pupil's' becomes 'OB's' (abbreviation for 'Old Boy' - private school pupil).
'remedy' becomes 'cure' (synonyms).
'obs'+'cure'='OBSCURE'
'is' is the link.
(Other definitions for obscure that I've seen before include "Dim, vague or not well known" , "Make unclear, indistinct, or blurred" , "Unsung" , "Hazy or indistinct" , "Not clear, hidden" .)