That one under the chestnut tree? (6)

I believe the answer is:

smithy

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'tree?' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for smithy that I've seen before include "Blacksmith's workplace" , "Forger's workshop" , "Where 3s were made and fitted" , "Vulcan was concerned?" , "Old forge" .)

I've seen this clue in the Evening Standard.

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