Crossword Genius

There are stacks of it out of town (3)

Ross

I believe the answer is:

hay

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'town' is the definition.
Although both the answer and definition are singular nouns, I can't understand 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 "Meadow crop" , "Cut and dried grass" , "Forage" , "Dried grass as fodder" , "Dried mown grass" .)

I've seen this clue in the Evening Standard.
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