Teacher back in prison (3)
I believe the answer is:
sir
'teacher' is the definition.
(a male teacher might be addressed as 'sir')
'back in prison' is the wordplay.
'back' says the letters should be written backwards.
'in' indicates the answer is hidden within the clue.
'ris' is hidden in the letters of 'prison'.
'ris' backwards is 'SIR'.
(Other definitions for sir that I've seen before include "Poet and novelist" , "Title given to baronet" , "Respectful address for a man" , "Male title of respect" , "Formal and deferential address to a man" .)