Screen student in cast (6)
I believe the answer is:
shield
'screen' is the definition.
(I know that screen can be written as shield)
'student in cast' is the wordplay.
'student' becomes 'l' (as in L-plates for learner drivers).
'in' means one lot of letters goes inside another.
'cast' becomes 'shied' (cast can mean to throw or shy).
'l' placed inside 'shied' is 'SHIELD'.
(Other definitions for shield that I've seen before include "Is held (anag.)" , "Is held as protective device" , "Sporting trophy; protect" , "Protect, screen" , "where you may find your bearings" .)