Cause the ruin of (4)

I think it could be either:

Which length description are you looking for?:

2,3

undo

'cause the ruin of' is the definition.
(I've seen this in another clue)

This is all the clue.

(Other definitions for undo that I've seen before include "Ruin or loose" , "Unfasten, loosen" , "Ruin, destroy" , "Open button or ruin" , "Open or unfasten" .)

I've seen this clue in The Times.

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