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 Penny Press, The Times, The Mirror, The Irish Independent, the Newsday, the Evening Standard, The Guardian and The New York Times.

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