Unexpected turn — gear ad due to show student (13)
I believe the answer is:
undergraduate
'student' is the definition.
(university student)
'unexpected turn gear ad due' is the wordplay.
'unexpected' indicates anagramming the letters.
'turn'+'gear'+'ad'+'due'='turngearaddue'
'turngearaddue' with letters rearranged gives 'UNDERGRADUATE'.
'to show' acts as a link.
(Other definitions for undergraduate that I've seen before include "One studying for a first degree" , "University student yet to take degree" .)