Former student caught in unmistakable disguise (7)
I believe the answer is:
obscure
'disguise' is the definition.
(thesaurus)
'former student caught in unmistakable' is the wordplay.
'former student' becomes 'ob' (abbreviation for 'Old Boy').
'caught' becomes 'c' (cricket abbreviation).
'in' means one lot of letters goes inside another.
'unmistakable' becomes 'sure'.
'c' inserted within 'sure' is 'scure'.
'ob'+'scure'='OBSCURE'
(Other definitions for obscure that I've seen before include "Like Jude, not well known" , "Enigmatic (like Jude?)" , "Insignificant" , "Hazy or cryptic" , "Unclear - recondite" .)