Null and void (7)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
invalid
'null and void' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for invalid that I've seen before include "Void; patient" , "Having no effect" , "to be in poor health" , "Disable" , "patient [found]" .)