Invalid (4)

I think it could be either:

void

null

'invalid' is the definition.
(similar in meaning)

This is the entire clue.

(Other definitions for void that I've seen before include "Empty or vacant, like space" , "Emptiness, empty space" , "Containing nothing or not legally binding" , "Blank, invalid" , "Empty space or vacuum" .)

I've seen this clue in The Mirror, the USA Today, The New York Times, the LA Times, the Newsday, The Irish Independent, the Penny Press, The Guardian, the Evening Standard and The Times.

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