Empty or unoccupied (6)
I believe the answer is:
vacant
I believe this is a double definition.
'empty' is the first definition.
(similar in meaning)
'unoccupied' is the second definition.
(synonyms)
(Other definitions for vacant that I've seen before include "Horse in a race" , "thoughtless" , "Not occupied, perhaps apartment" , "Empty - stupid" , "Of house, not occupied at present" .)