Marsh plant in small border (5)
I believe the answer is:
sedge
'marsh plant' is the definition.
(sedge is a kind of marsh plant)
'small border' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'border' becomes 'edge' ('edge' can be a synonym of 'border').
's'+'edge'='SEDGE'
'in' is the link.
(Other definitions for sedge that I've seen before include "plant with triangular stems" , "Coarse, grasslike plant" , "Herons" , "Waterside plant" , "Wetland grass-like plant" .)