He backs a horse (7)
I believe the answer is:
saddler
'he backs a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'horse' is the wordplay.
I cannot quite see how this works, but
'horse' could be 'saddle' (saddle horse is a kind of horse) and 'saddle' is found in the answer.
A single letter 'r' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for saddler that I've seen before include "he'll offer you a seat" , "Maker of seats for riders" , "I'll make seat" , "Leather-worker" , "One makes leather goods for horses" .)