Gentleman back in prison (3)
I believe the answer is:
sir
'gentleman' is the definition.
(a man may be addressed as sir)
'back in prison' is the wordplay.
'back' is a reversal indicator.
'in' indicates a hidden word.
'ris' is hidden within 'prison'.
'ris' back-to-front is 'SIR'.
(Other definitions for sir that I've seen before include "Title of 1 across" , "Title of knight or baronet" , "Title of Kt or Bart" , "Poet and novelist" , "Form of address for a knight" .)