Mule, perhaps one in a horse race (6)
I believe the answer is:
runner
This might be a double definition.
'mule' is the first definition.
The answer and definition can be both animals as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'one in a horse race' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'perhaps' is the link.
Can you help me to learn more?
(Other definitions for runner that I've seen before include "Athlete - mat - bean" , "Horse (in a race); bean plant" , "Horse in a race; shoot" , "Athlete; narrow rug" , "Athlete; messenger" .)