Margin of the map (3)
I believe the answer is:
hem
'margin' is the definition.
(I've seen this in another clue)
'of the map' is the wordplay.
'of' says the answer is hidden in the clue.
'HEM' is hidden in the letters of 'the map'.
(Other definitions for hem that I've seen before include "may attract attention" , "Double-backed edge" , "Finished edge of a piece of cloth" , "Part of skirt" , "Sewn border" .)