Borders formed of sedge (5)
I believe the answer is:
edges
'borders' is the definition.
(an edge is a border or margin)
'of sedge' is the wordplay.
'of' is an anagram indicator (the answer comes from or 'of' the letters).
'sedge' with letters rearranged gives 'EDGES'.
'formed' acts as a link.
(Other definitions for edges that I've seen before include "Des, e.g. (anag.)" , "Fringes" , "Outside limits" , "They're farthest from the middle" , "Boundaries of a surface" .)