Ruling house of Monaco (8)
I believe the answer is:
grimaldi
'ruling house of monaco' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for grimaldi that I've seen before include "Pantomime actor" , "Joseph -, eighteenth to nineteenth century English clown" , "English clown's" , "Genoese family" , "Ruling house of Monaco" .)