Student’s unusually guarded nature (13)
I believe the answer is:
undergraduate
'student's' is the definition.
(undergraduate is a kind of student)
'unusually guarded nature' is the wordplay.
'unusually' indicates an anagram.
'guarded'+'nature'='guardednature'
'guardednature' with letters rearranged gives 'UNDERGRADUATE'.
(Other definitions for undergraduate that I've seen before include "One studying for a first degree" , "University student yet to take degree" .)