No term arranged for teacher (6)
'teacher' is the definition.
(a mentor is a teacher or guide)
'no term arranged' is the wordplay.
'arranged' is an anagram indicator.
'noterm' anagrammed gives 'MENTOR'.
'for' is the link.
(Other definitions for mentor that I've seen before include "Merton (anag.)", ""Guide, philosopher, and friend"", "Guru", "No term for instructor", "A trusted adviser or teacher".)