Evil king's decline (4)
I believe the answer is:
sink
'decline' is the definition.
(sinking is a kind of declining)
'evil king's' is the wordplay.
'evil' becomes 'sin' (I've seen this before).
'king' becomes 'k'.
'sin'+'k'='SINK'
(Other definitions for sink that I've seen before include "Submerge; basin" , "Go down (in the kitchen?)" , "Fall to the bottom" , "Drop in the ocean?" , "Washing-up place" .)