Gentleman has time inside prison (4)
I believe the answer is:
stir
'prison' is the definition.
(informal term for prison)
'gentleman has time' is the wordplay.
'gentleman' becomes 'sir' (I've seen this before).
'has' is an insertion indicator.
'time' becomes 't'.
'sir' going around 't' is 'STIR'.
'inside' acts as a link.
(Other definitions for stir that I've seen before include "created by an agitator?" , "in which you'll find people with convictions" , "Ado" , "Cause trouble" , "Agitate, arouse" .)