Crossword Genius

Nullifies horrendous ultimate chaos! (6)

Ross

I believe the answer is:

undoes

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'nullifies' is the definition.
(I've seen this in another clue)

I don't understand the rest of the clue.

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(Other definitions for undoes that I've seen before include "Opens fastener or causes the ruin of" , "Loosens or ruins" .)

I've seen this clue in the Evening Standard.
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