Let no student relax (4)
I believe the answer is:
ease
'relax' is the definition.
(I've seen this before)
'let no student' is the wordplay.
'let' becomes 'lease' (as in to lease or let a building).
'no' is a deletion indicator.
'student' becomes 'l' (as in L-plates for learner drivers).
'lease' with 'l' removed is 'EASE'.
(Other definitions for ease that I've seen before include "composure" , "Absence of difficulty" , "Comfort; facility" , "Lessen intensity" , "Leisure, comfort" .)