Teacher back in garrison (3)
I believe the answer is:
sir
'teacher' is the definition.
(a male teacher might be addressed as 'sir')
'back in garrison' is the wordplay.
'back' says the letters should be written backwards.
'in' says the answer is hidden in the clue.
'ris' is hidden in the letters of 'garrison'.
'ris' back-to-front is 'SIR'.
(Other definitions for sir that I've seen before include "Polite or formal address to male" , "Title given to 10 Across" , "Polite term of address for a man" , "Male title of respect" , "Baronet's title" .)