Prison time breaks a gentleman (4)
I believe the answer is:
stir
'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 "song and dance" , "Act as an agitator" , "created by an agitator?" , "fuss" , "Wake up - prison" .)