Former pupil's remedy is hard to understand (7)
I believe the answer is:
obscure
'hard to understand' is the definition.
(thesaurus)
'former pupil's remedy' is the wordplay.
'former pupil's' becomes 'OB's' (abbreviation for 'Old Boy').
'remedy' becomes 'cure' (synonyms).
'obs'+'cure'='OBSCURE'
'is' acts as a link.
(Other definitions for obscure that I've seen before include "Little-known" , "Gloomy" , "Arcane" , "unimportant" , "Unsung" .)