There's a street market for it in London (3)
I believe the answer is:
hay
'london' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the 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 "It is to be made while the sun shines" , "Welsh festival" , "basic bedding" , "Straw" , "Animal fodder" .)