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"nullify" is also used as a crossword clue. If that's what you saw, the answer is often invalidate (10 letters).
Definitions
make ineffective by counterbalancing the effect of
Word origin
From null + -ify.
Used in a sentence
“Near-synonyms: cancel, void, quash”
“The contract has been nullified.”
“[T]hey were baffled by tears in moustached sixth-formers, by walls of impassivity in the Lower School, by silent conspiracies which nullified the rules.”
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer1 curated clues
01“Render void”7 letters
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