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Shortly, you'll find it next to London on the map (5)

Ross

I believe the answer is:

derry

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'map' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

I cannot understand how the rest of the clue works.

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(Other definitions for derry that I've seen before include "Port" , "Ulster city" , "'The London - - Air' (mus.)" .)

I've seen this clue in the Evening Standard.
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