Stop student in headgear (4)
I believe the answer is:
halt
'stop' is the definition.
(halt can mean to stop)
'student in headgear' is the wordplay.
'student' becomes 'l' (as in L-plates for learner drivers).
'in' means one lot of letters goes inside another.
'headgear' becomes 'hat' (hat is a kind of headgear).
'l' going within 'hat' is 'HALT'.
(Other definitions for halt that I've seen before include "Stop; minor station" , "Cease" , "Falter - stop" , "Pull up" , "Stop or pause" .)