In fact, try out a superior student (5)
I believe the answer is:
truly
'in fact' is the definition.
(I've seen this in another clue)
'try out a superior student' is the wordplay.
'out' means one lot of letters goes inside another.
'a superior' becomes 'U' (U can mean upper-class, as in U and non-U speech).
'student' becomes 'L' (learner, as in L-plates for learner drivers).
'u'+'l'='ul'
'try' going around 'ul' is 'TRULY'.
(Other definitions for truly that I've seen before include "Really, genuinely" , "Honestly" , "Indeed" , "In reality" , "Verily" .)