Bring to naught (4)
I believe the answer is:
undo
'bring to naught' 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 "Loosen; annul" , "Bring to nought" , "Loosen (knot)" , "Cause downfall" , "Bring to ruin" .)