Supreme soldier sitting on horse (9)
I believe the answer is:
paramount
'supreme' is the definition.
(I've seen this before)
'soldier sitting on horse' is the wordplay.
'soldier' becomes 'para' (paratrooper).
'sitting on' says to put letters next to each other.
'horse' becomes 'mount' (mount is a kind of horse).
'para'+'mount'='PARAMOUNT'
(Other definitions for paramount that I've seen before include "outstanding" , "Of overriding importance" , "Of the greatest importance" , "Superior to all others" , "First" .)