Old fashioned garden in small area (6,4)

I believe the answer is:

square yard

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

'old fashioned garden' is the wordplay.
'old fashioned' becomes 'square'.
'garden' becomes 'yard' (I have seen 'American garden' mean 'yard' so perhaps 'garden' could also mean 'yard').
'square'+'yard'='SQUARE YARD'

'in' is the link.

Can you help me to learn more?

I've seen this clue in The Guardian.

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