Teacher to express disapproval otherwise (5)
I believe the answer is:
tutor
'teacher' is the definition.
(tutor is a kind of teacher)
'express disapproval otherwise' is the wordplay.
'express disapproval' becomes 'tut' (I've seen this in other clues).
'otherwise' becomes 'or'.
'tut'+'or'='TUTOR'
'to' acts as a link.
(Other definitions for tutor that I've seen before include "trainer" , "Give individual instruction" , "Individual teacher" , "Edify" , "college teacher" .)