Teacher in toupee retired at university (4)
I believe the answer is:
guru
'teacher' is the definition.
(teacher or guide)
'toupee retired at university' is the wordplay.
'toupee' becomes 'rug' (rug is a kind of toupee).
'retired' shows that the letters should be reversed in order.
'at' says to put letters next to each other.
'university' becomes 'U' (abbreviation for university).
'rug' in reverse letter order is 'gur'.
'gur'+'u'='GURU'
'in' acts as a link.
(Other definitions for guru that I've seen before include "Authoritative source" , "Name given to the each of the first ten leaders of the Sikh religion" , "Religious leader and spiritual teacher" , "Spiritual guide" , "Revered mentor" .)