Where's Douglas? (4,2,3)

I believe the answer is:

isle of man

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'douglas?' is the definition.
Both the definition and answer are singular nouns.
Perhaps there's an association between them I don't understand?

I can't explain the remainder of the clue.

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(Other definitions for isle of man that I've seen before include "Its capital is Douglas and its cats are tailless" , "Here are Peel, Douglas" , "Island in the Irish Sea" , "British outpost of the Gaels" , "TT venue" .)

I've seen this clue in the Evening Standard.

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