Evil king to fall (4)
I believe the answer is:
sink
'fall' is the definition.
(I know that sink is a type of drop)
'evil king' is the wordplay.
'evil' becomes 'sin' (synonyms).
'king' becomes 'K' (abbreviation for king in chess).
'sin'+'k'='SINK'
'to' acts as a link.
(Other definitions for sink that I've seen before include "(Cause to) become submerged" , "Go down; wash basin" , "Excavate (a well)" , "Washing facility" , "Plumbing fixture" .)