He sells at a knock-down price (10)
I believe the answer is:
auctioneer
'he sells' is the definition.
(an auctioneer facilitates sales at auction)
'a knock-down price' is the wordplay.
I cannot quite see how this works, but
'a' could be 'one' and 'one' is present in the answer.
This may be the basis of clue (or it may be nonsense).
'at' acts as a link.
(Other definitions for auctioneer that I've seen before include "one that has lots to offer" , "One inviting bids for lots" , "One selling lots" .)