Former pupil's recovery is uncertain (7)
I believe the answer is:
obscure
'uncertain' is the definition.
(obscure can mean vague or uncertain)
'former pupil's recovery' is the wordplay.
'former pupil's' becomes 'OB's' (abbreviation for 'Old Boy').
'recovery' becomes 'cure' (as in recovery from illness).
'obs'+'cure'='OBSCURE'
'is' acts as a link.
(Other definitions for obscure that I've seen before include "Little known - dark" , "Make unclear, indistinct, or blurred" , "Dull or impenetrable" , "Not distinct" , "Not clear, indistinct" .)