Cast in support of sitting ruler, by the sound of it (6)
I believe the answer is:
thrown
'cast' is the definition.
(cast can mean to throw)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for thrown that I've seen before include "Defeated" , "Parties are" , "Hurled, flung" , "Confused - cast" , "Unseated" .)