Simple map I enclosed (5)

I believe the answer is:

plain

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'simple' is the definition.
('plain' can be a synonym of 'simple')

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

(Other definitions for plain that I've seen before include "patent" , "Evident; not fancy" , "Not complicated" , "lament" , "Not elaborate; clear" .)

I've seen this clue in The Telegraph.

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