Registration of mentor Len upset (9)
I believe the answer is:
enrolment
'registration' is the definition.
(eg enrolling at a university)
'mentor len upset' is the wordplay.
'upset' indicates anagramming the letters.
'mentor'+'len'='mentorlen'
'mentorlen' is an anagram of 'ENROLMENT'.
'of' acts as a link.
(Other definitions for enrolment that I've seen before include "matriculation" , "signing up" , "Act of entering on a list of members" , "Joining" , "Registration" .)