Small border plant (5)
I believe the answer is:
sedge
'plant' is the definition.
(sedge is a plant)
'small border' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'border' becomes 'edge' (synonyms).
's'+'edge'='SEDGE'
(Other definitions for sedge that I've seen before include "Marsh-dweller" , "Herons" , "Wetland grass-like plant" , "Marsh grass" , "Plant growing in swampy ground" .)