Marsh plant with small border (5)
I believe the answer is:
sedge
'marsh plant' is the definition.
(sedge is a kind of marsh plant)
'with small border' is the wordplay.
'with' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'border' becomes 'edge' (synonyms).
's' put next to 'edge' is 'SEDGE'.
(Other definitions for sedge that I've seen before include "plant with triangular stems" , "Plant growing in swampy ground" , "Coarse, grasslike plant" , "Bog plant; type of warbler" , "Marsh-dweller" .)