Fool covering king in some butter (4)
I believe the answer is:
prat
'fool' is the definition.
(I know that fool can be written as prat)
'covering king in some butter' is the wordplay.
'covering king' becomes 'r' (abbreviation for rex, king in Latin. I am not sure about the 'covering' bit.).
'in' indicates putting letters inside.
'some butter' becomes 'pat' (a pat can mean a lump of butter).
'r' going within 'pat' is 'PRAT'.
(Other definitions for prat that I've seen before include "Incompetent idiot" , "Wally" , "Stupid person" , "Rapt like silly ass" , "Rapt kind of fool" .)