There's a market for it in a London street (3)
I believe the answer is:
hay
'street' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for hay that I've seen before include "Dried mown grass" , "Animal fodder" , "Straw" , "Welsh festival" , "Forage" .)