Such a contractor may quote a knock-down price (10)
I believe the answer is:
demolition
'price' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for demolition that I've seen before include "destructive action" , "explosion" , "Licking" , "Knocking down" , "defeat" .)