Pain concealed by teacher (4)
I believe the answer is:
ache
'pain' is the definition.
(ache is a kind of pain)
'concealed by teacher' is the wordplay.
'concealed by' indicates the answer is hidden within the clue.
'ACHE' can be found hidden inside 'teacher'.
(Other definitions for ache that I've seen before include "Dull steady pain" , "Nagging pain" , "A dull lingering pain" , "Dull continuous pain" , "Dull throbbing pain" .)