Bog plant in small border (5)
'bog plant' is the definition.
(sedge is a kind of bog plant)
'small border' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'border' becomes 'edge' (synonyms).
'in' is the link.
(Other definitions for sedge that I've seen before include "Grasslike marsh plant", "Grass-like plant growing in marshes", "Coarse grassy plant", "Herons", "Wetland grass-like plant".)