Sit with Queen on carthorse (9)
I believe the answer is:
percheron
'carthorse' is the definition.
(I've seen this before)
'sit with queen on' is the wordplay.
'sit' becomes 'perch' (perching is a kind of sitting**).
'with' says to put letters next to each other.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'perch'+'er'+'on'='PERCHERON'
(Other definitions for percheron that I've seen before include "heavy horse" , "draught horse" , "Carthorse" .)