Unoccupied or not in use (4)
I believe the answer is:
idle
I believe this clue is a double definition.
'unoccupied' is the first definition.
(similar in meaning)
'not in use' is the second definition.
(I've seen this in another clue)
(Other definitions for idle that I've seen before include "loiter aimlessly" , "Monty Python team member" , "Disliking work" , "not working in > lounge" , "Futile" .)