Spilled ink on good ruler (4)
I believe the answer is:
king
'ruler' is the definition.
(I know that king is a type of ruler)
'spilled ink on good' is the wordplay.
'spilled' indicates anagramming the letters.
'on' says to put letters next to each other.
'good' becomes 'g' (abbreviation).
'ink' anagrammed gives 'kin'.
'kin'+'g'='KING'
(Other definitions for king that I've seen before include "Chess piece" , "initially unable to move" , "Court card" , "US civil rights leader, assassinated 1968" , "Male on the throne" .)