Express disapproval over heartless teacher (5)
I believe the answer is:
tutor
'teacher' is the definition.
(tutor is a kind of teacher)
'express disapproval over heartless' is the wordplay.
'express disapproval' becomes 'tut' (I've seen this in other clues).
'heartless' suggests removing the centre.
'over' with its middle removed is 'or'.
'tut'+'or'='TUTOR'
(Other definitions for tutor that I've seen before include "Mentor" , "He shows us the way" , "college teacher" , "Private educator" , "Teacher (often private)" .)