Plant forms small border (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'
'forms' acts as a link.
(Other definitions for sedge that I've seen before include "Grasslike marsh plant" , "Herons" , "Coarse grassy plant" , "Coarse, grasslike plant" , "Waterside plant" .)