Former pupil's remedy is uncertain (7)
I believe the answer is:
obscure
'uncertain' is the definition.
(obscure can mean vague or uncertain)
'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 "Make unclear, indistinct, or blurred" , "Not clearly expressed" , "Dim, vague or not well known" , "Little-known" , "Unheard-of" .)