Sum of money for a steed (6)
I believe the answer is:
amount
'sum of money' is the definition.
(synonyms)
'a steed' is the wordplay.
'steed' becomes 'mount' (I've seen this in another clue).
'a'+'mount'='AMOUNT'
'for' acts as a link.
(Other definitions for amount that I've seen before include "Sum of money" , "Total or extent" , "A quantity or total of anything" , "Quantity, sum" , "Supply" .)