Pain in teacher's heart (4)
I believe the answer is:
ache
'pain' is the definition.
(ache is a kind of pain)
'in teacher's heart' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ACHE' is hidden within 'teachers heart'.
(Other definitions for ache that I've seen before include "Continuous hurt" , "Cause pain" , "burn" , "agony" , "Dull prolonged pain" .)