King wants foul fiend stabbed (6)
I believe the answer is:
knifed
'stabbed' is the definition.
('knife' can be a synonym of 'stab')
'king wants foul fiend' is the wordplay.
'king' becomes 'k'.
'wants' says to put letters next to each other.
'foul' indicates anagramming the letters.
'fiend' with letters rearranged gives 'nifed'.
'k'+'nifed'='KNIFED'
(Other definitions for knifed that I've seen before include "Cut" , "deceived furtively" , "Stabbed with intent to harm or kill" .)