Supreme soldier's horse (9)
I believe the answer is:
paramount
'supreme' is the definition.
(I've seen this before)
'soldier's horse' is the wordplay.
'soldier' becomes 'para' (para is a kind of soldier).
'horse' becomes 'mount' (mount is a kind of horse).
'para'+'mount'='PARAMOUNT'
(Other definitions for paramount that I've seen before include "Sovereign" , "More important than anything else" , "Supreme - Hollywood studio" , "Of overriding importance" , "Chief in importance, supreme" .)