Most important soldier on horseback (9)
I believe the answer is:
paramount
'most important' is the definition.
(I've seen this in another clue)
'soldier on horseback' is the wordplay.
'soldier' becomes 'para' (short for paratrooper).
'on horseback' becomes 'mount' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'para'+'mount'='PARAMOUNT'
Can you help me to learn more?
(Other definitions for paramount that I've seen before include "Superior to all others" , "More important than anything else" , "Chief in importance, supreme" , "Of overriding importance" , "Highest" .)