Unseated by horse, one the Queen’s sat on, reportedly? (6)
I believe the answer is:
thrown
'unseated by horse' is the definition.
'one the queen's sat on reportedly?' is the wordplay.
'one the queen's sat on' becomes 'throne'.
'reportedly?' indicates a 'sounds like' (homophone) clue (I've seen 'reportedly' mean this).
'throne' is a homophone of 'THROWN'.
(Other definitions for thrown that I've seen before include "Parties are" , "Hurled, flung" , "given up intentionally" , "Having been projected through the air" , "Confused; chucked" .)