Measure a horse (6)
I believe the answer is:
amount
'measure' is the definition.
(synonyms)
'a horse' is the wordplay.
'horse' becomes 'mount' (mount is a kind of horse).
'a'+'mount'='AMOUNT'
(Other definitions for amount that I've seen before include "Quantity, sum" , "Size" , "Measure, quantity" , "A quantity or total of anything" , "Total or extent" .)