Cause the ruin of (4)
I believe the answer is:
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 "Unravel or ruin" , "Ruin or loose" , "Open or unfasten" , "Open button or ruin" , "Unfasten or annul" .)