Dealer treated a horse well (10)
I believe the answer is:
wholesaler
'dealer' is the definition.
(I know that wholesaler is a type of dealer)
'treated a horse well' is the wordplay.
'treated' indicates an anagram.
'a'+'horse'+'well'='ahorsewell'
'ahorsewell' with letters rearranged gives 'WHOLESALER'.
(Other definitions for wholesaler that I've seen before include "Merchant" , "Bulk trader" , "firm that supplies in bulk" , "Supplier to the trade" , "Retailer's supplier" .)