The maps always show this border (3)
I believe the answer is:
hem
'this border' is the definition.
(I've seen this before)
'the maps always show' is the wordplay.
'show' says the answer is hidden in the clue.
'HEM' is hidden within 'the maps always'.
(Other definitions for hem that I've seen before include "may attract attention" , "Double-backed border of a garment" , "A stitched border of cloth" , "try to attract attention" , "Bottom of a garment" .)