Empty container found outside jail (6)
I believe the answer is:
vacant
'empty' is the definition.
(similar in meaning)
'container found outside jail' is the wordplay.
'container' becomes 'vat' (vat is a kind of container).
'found outside' indicates putting letters inside.
'jail' becomes 'can' (the can is American slang for prison).
'vat' going around 'can' is 'VACANT'.
(Other definitions for vacant that I've seen before include "Unoccupied - unthinking" , "Empty, not occupied" , "available" , "... Blank" , "thoughtless" .)