Bird riding big horse (9)
I believe the answer is:
percheron
'big horse' is the definition.
(breed of horse)
'bird riding' is the wordplay.
'bird' becomes 'percher' (something which perches).
'riding' becomes 'on' (I've seen this in other clues).
'percher'+'on'='PERCHERON'
(Other definitions for percheron that I've seen before include "draught horse" , "Carthorse" , "heavy horse" .)