Possibly arming border (6)
I believe the answer is:
margin
'border' is the definition.
(I know that border can be written as margin)
'possibly arming' is the wordplay.
'possibly' indicates anagramming the letters.
'arming' with letters rearranged gives 'MARGIN'.
(Other definitions for margin that I've seen before include "Blank space that surrounds the text on a page" , "Blank space around a text" , "Edge or space for error" , "Blank space around a page" , "Space at edge of page" .)