Prison time breaks a gentleman (4)
I believe the answer is:
Here is my best explanation:
'prison' is the definition.
(informal term for prison)
'time breaks a gentleman' is the wordplay.
'time' becomes 't'.
'breaks' means one lot of letters goes inside another.
'a gentleman' becomes 'sir' (I've seen this before).
't' placed within 'sir' is 'STIR'.
(Other definitions for stir that I've seen before include "move very slightly, or agitate", "Wake; disturbance", "Go about", "Bustle, activity", "Slight movement", "Awaken", "fuss", "Thrill", "More very slightly, or agitate" and "Nick".)