Free of charge perhaps (5)
I believe the answer is:
empty
'free' is the definition.
(both can mean unoccupied)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for empty that I've seen before include "Containing nothing" , "(Of words) insincere" , "Devoid of content" , "Cleared out" , "Dead man" .)