A horse, as in an instance of backing? (5)
I believe the answer is:
beast
'a horse' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Maybe there's an association between them I don't understand?
'as in an instance of backing?' is the wordplay.
'in' means one lot of letters goes inside another.
'an instance of backing?' becomes 'bet' (I can't explain this - if you can you should believe this answer much more).
'as' put into 'bet' is 'BEAST'.
Can you help me to learn more?
(Other definitions for beast that I've seen before include "Inhumanly cruel person" , "rotter" , "Savage" , "Brute; creature" , "Pig" .)