Crossword Genius

Country product on the market in London (3)

Ross

I believe the answer is:

hay

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'london' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

I don't understand the remainder of the clue.

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(Other definitions for hay that I've seen before include "Wye book town" , "Animal fodder" , "It is to be made while the sun shines" , "Cut and dried grass" , "Dried mown grass" .)

I've seen this clue in the Evening Standard.
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