Supply a horse for riding (6)
I believe the answer is:
amount
'supply' is the definition.
(I've seen this before)
'a horse for riding' is the wordplay.
'horse for riding' becomes 'mount' (horse that can be mounted).
'a'+'mount'='AMOUNT'
(Other definitions for amount that I've seen before include "Total or extent" , "A quantity or total of anything" , "Quantity, sum" , "value" , "Size" .)