One of two footholds on the mount (7)
I believe the answer is:
stirrup
This could be a double definition.
'one of two footholds' is the first definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'the mount' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'on' is the link.
Can you help me to learn more?
(Other definitions for stirrup that I've seen before include "Rider's foot rest" , "Horse rider's footrest" , "Foot support in riding" , "Foot rest for a rider" , "Rider's foot loop" .)