Ruin, downfall (7)
I believe the answer is:
undoing
This clue is a double definition.
'ruin' is the first definition.
(I've seen this before)
'downfall' is the second definition.
(both can mean a loss of power or position)
(Other definitions for undoing that I've seen before include "Cause of ruin" , "Taking apart" , "(A person's) ruin or downfall" , "Ruin - disaster" .)