Teacher's wig sent up by third of students (4)
I believe the answer is:
guru
'teacher's' is the definition.
(teacher or guide)
'wig sent up by third of students' is the wordplay.
'wig' becomes 'rug' (rug is a slang term for a wig).
'sent up' says the letters should be written backwards.
'by' says to put letters next to each other.
'third of students' becomes 'u' (3rd letter of 'students').
'rug' in reverse letter order is 'gur'.
'gur'+'u'='GURU'
(Other definitions for guru that I've seen before include "Influential expert" , "Hindu or Buddhist spiritual teacher" , "Recognised leader in some field" , "Swami" , "Wise leader" .)