Student exposed as yob (4)
I believe the answer is:
lout
'yob' is the definition.
(both can mean an antisocial young man)
'student exposed' is the wordplay.
'student' becomes 'L' (as in L-plates for learner drivers).
'exposed' becomes 'out' (to out someone is to expose them).
'l'+'out'='LOUT'
'as' acts as a link.
(Other definitions for lout that I've seen before include "Antisocial person" , "A noisy, badly-behaved youth" , "A noisy coarse youth" , "Yahoo" , "'Rude, ill-mannered fellow (4)'" .)