Evil ruler going under (7)
I believe the answer is:
sinking
'going under' is the definition.
('sink' can be a synonym of 'go under')
'evil ruler' is the wordplay.
'evil' becomes 'sin' (synonyms).
'ruler' becomes 'king' (king is a kind of ruler).
'sin'+'king'='SINKING'
(Other definitions for sinking that I've seen before include "Falling to the bottom" , "Declining" , "in decline" , "Going down" , "failing" .)