Two garments in one? (5,4)

I believe the answer is:

frock coat

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

'two garments' is the wordplay.
'two garments' means two substitutions for 'garment'.
'garment' becomes 'frock' (frock is a kind of garment).
'garment' becomes 'coat' (coat is a kind of garment).
'frock'+'coat'='FROCK-COAT'

'in' acts as a link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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