A good one gets a lot of backing (9)
I believe the answer is:
racehorse
'a good' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'one gets a lot of backing' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'ace' (ace has a value of one in some card games) and 'ace' is found within the answer.
'of' could be 'o' and 'o' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for racehorse that I've seen before include "Animal" , "Steeplechaser, maybe" , "hope for better?" , "Arab possibly" , "Which a jockey rides" .)