Value of American horse (6)
I believe the answer is:
'value' is the definition.
(I've seen this before)
'american horse' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'horse' becomes 'mount' (mount is a kind of horse).
'of' is the link.
(Other definitions for amount that I've seen before include "Add up or be equivalent to" , "Quantity, sum" , "Size" , "A quantity or total of anything" , "Supply" .)