Doctored main polo mount (8)
I believe the answer is:
palomino
'mount' is the definition.
(palomino is a type of horse)
'doctored main polo' is the wordplay.
'doctored' is an anagram indicator.
'main'+'polo'='mainpolo'
'mainpolo' with letters rearranged gives 'PALOMINO'.
(Other definitions for palomino that I've seen before include "Equine" , "White-maned horse of the southwest USA" , "colt?" , "Golden horse with white mane and tail" , "Colour breed of horse" .)