What one behind bars called the fourth dimension (4)
I believe the answer is:
time
I believe this is a double definition.
'what one behind bars called' is the first definition.
(when alcohol is no longer being served)
'the fourth dimension' is the second definition.
('time' can be a synonym of 'fourth dimension')
(Other definitions for time that I've seen before include "Bird" , "Hours clocked up?" , "Its detonation is delayed" , ""The enemy"" , "Procrastination is the thief of ." .)