First soldier, high one on a horse (8)
I believe the answer is:
stallion
'a horse' is the definition.
(I know that stallion is a type of Horse)
'first soldier high one on' is the wordplay.
'first soldier' becomes 's' (tst letter of 'soldier').
'high' becomes 'tall' (similar in meaning).
'one' becomes 'I' (Roman numeral).
's'+'tall'+'i'+'on'='STALLION'
(Other definitions for stallion that I've seen before include "Stud" , "Mare's mate" , "Mount" , "Male steed" , "Male horse, often used for breeding" .)