Special border plant (5)
I believe the answer is:
sedge
'plant' is the definition.
The answer is an example.
'special border' is the wordplay.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'border' becomes 'edge' (synonyms).
's'+'edge'='SEDGE'
(Other definitions for sedge that I've seen before include "Marsh-dweller" , "Coarse grassy plant" , "Herons" , "fly by rivers" , "Marsh grass" .)