Sum provided for a horse? (6)
I believe the answer is:
amount
'sum' is the definition.
(I know that sum can be written as amount)
'a horse?' is the wordplay.
'horse?' becomes 'mount' (mount is a kind of horse).
'a'+'mount'='AMOUNT'
'provided for' acts as a link.
(Other definitions for amount that I've seen before include "A quantity or total of anything" , "Sum of money" , "Quantity, sum" , "value" , "Add up or be equivalent to" .)