Prison time breaks a gentleman (4)
I believe the answer is:
Here is my best explanation:
'prison' is the definition.
(I've seen this before)
'time breaks a gentleman' is the subsidiary indication.
'time' becomes 't'.
'breaks' is an insertion indicator.
'a gentleman' becomes 'sir' (I've seen this before).
't' inserted into 'sir' is 'stir'.
(Other definitions for stir that I've seen before include "move very slightly, or agitate", "Wake; disturbance", "College", "Bustle, activity", "Slight movement", "Awaken", "fuss", "bird", "More very slightly, or agitate" and "Nick".)