What it costs to have a horse? (6)
I believe the answer is:
amount
'what it costs to' is the definition.
I can't tell whether this defines the answer.
'a horse?' is the wordplay.
'horse?' becomes 'mount' (mount is a kind of horse).
'a'+'mount'='AMOUNT'
'have' acts as a link.
Can you help me to learn more?
(Other definitions for amount that I've seen before include "value" , "Supply" , "Add up or be equivalent to" , "Size" , "A quantity or total of anything" .)