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Diagrammatically, London's thin red line (7)

Ross

I believe the answer is:

central

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'line' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see an association between them that I can't see?

I cannot understand how the rest of the clue works.

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(Other definitions for central that I've seen before include "Extremely important" , "Every answer here has one such letter" , "Key" , "like this answer" , "Most important, principal" .)

I've seen this clue in the Evening Standard.
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