Quantity suitable for a horse (6)
I believe the answer is:
'quantity' is the definition.
(I know that quantity can be written as amount)
'a horse' is the wordplay.
'horse' becomes 'mount' (mount is a kind of horse).
'suitable for' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for amount that I've seen before include "Sum of money" , "Total or extent" , "value" , "A quantity or total of anything" , "Size" .)