Boundary, margin (6)
I believe the answer is:
border
I believe this is a double definition.
'boundary' is the first definition.
(I know that border is a type of boundary)
'margin' is the second definition.
(I know that margin can be written as border)
(Other definitions for border that I've seen before include "Rim" , "Line that indicates a boundry" , "Type of sheepdog" , "BLINK/BRINK" , "Quantity; song" .)