Tell a student to be relaxed (8)
I believe the answer is:
informal
'be relaxed' is the definition.
(thesaurus)
'tell a student' is the wordplay.
'tell' becomes 'inform' (I've seen this before**).
'student' becomes 'l' (as in L-plates for learner drivers).
'inform'+'a'+'l'='INFORMAL'
'to' acts as a link.
(Other definitions for informal that I've seen before include "Casual and unceremonious" , "Unofficial, relaxed" , "Cosy -- loose" , "Casual or unofficial" , "Casual, not official" .)