Vacuum (4)
I believe the answer is:
void
'vacuum' is the definition.
(I know that vacuum can be written as void)
This is the entire clue.
(Other definitions for void that I've seen before include "Not valid or legally binding" , "Empty or vacant, like space" , "Empty space or not legally binding" , "Containing nothing or not legally binding" , "Emptiness, empty space" .)