Changing main polo mount (8)
I believe the answer is:
palomino
'mount' is the definition.
(palomino is a type of horse)
'changing main polo' is the wordplay.
'changing' indicates an anagram.
'main'+'polo'='mainpolo'
'mainpolo' is an anagram of 'PALOMINO'.
(Other definitions for palomino that I've seen before include "colt?" , "Horse bred in SW US" , "Colour breed of horse" , "White-maned horse of the southwest USA" , "Equine" .)