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