Marks way by road from Edinburgh to London, north to south (6)

I believe the answer is:

stains

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'marks' is the definition.
('stain' can be a synonym of 'mark')

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

Can you help me to learn more?

(Other definitions for stains that I've seen before include "Discolourations on satins" , "Discolorations or blemishes" , "Sullies" , "Discolorations or dirt marks" , "Discolours or dyes wood" .)

I've seen this clue in The Guardian.

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