Bring to ruin (4)
I believe the answer is:
undo
'bring to ruin' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for undo that I've seen before include "Open buttons or ruin" , "Ruin prospects of; cancel" , "Reverse effect of" , "Relax" , "Untie or ruin" .)