Evil king's ruin (4)
I believe the answer is:
sink
'ruin' is the definition.
(thesaurus)
'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 "something needed in kitchen" , "Drop in the ocean?" , "place of corruption" , "Invest capital in - kitchen basin - founder" , "Droop, submerge" .)