Express disapproval: our heartless teacher! (5)
I believe the answer is:
tutor
'teacher' is the definition.
(tutor is a kind of teacher)
'express disapproval our heartless' is the wordplay.
'express disapproval' becomes 'tut' (I've seen this in other clues).
'heartless' means to remove the middle letters.
'our' with its centre removed is 'or'.
'tut'+'or'='TUTOR'
(Other definitions for tutor that I've seen before include "class leader" , "train" , "Master" , "Guardian" , "Mentor" .)