A horse for a price (6)
I believe the answer is:
amount
'price' is the definition.
(thesaurus)
'a horse for a' is the wordplay.
'a horse' becomes 'mount' (mount is a kind of horse).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'mount' after 'a' is 'AMOUNT'.
(Other definitions for amount that I've seen before include "Measure, quantity" , "Size" , "Add up or be equivalent to" , "Total or extent" , "Sum of money" .)