Teacher back in prison (3)
I believe the answer is:
sir
'teacher' is the definition.
(way of addressing a male teacher)
'back in prison' is the wordplay.
'back' says the letters should be written in reverse.
'in' indicates a hidden word.
'ris' is hidden in the letters of 'prison'.
'ris' back-to-front 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" .)