Student no longer in thrall to enthusiast (5)
I believe the answer is:
lover
'enthusiast' is the definition.
(thesaurus)
'student no longer in thrall to' is the wordplay.
'student' becomes 'l' (as in L-plates for learner drivers).
'no longer in thrall to' becomes 'over' (as in getting over a former lover).
'l'+'over'='LOVER'
(Other definitions for lover that I've seen before include "finished litre first" , "Paramour" , "Swain" , "One of a couple" , "fan" .)