Plant in southern border (5)
I believe the answer is:
sedge
'plant' is the definition.
(sedge is a plant)
'southern border' is the wordplay.
'southern' becomes 's' (abbreviation).
'border' becomes 'edge' (synonyms).
's'+'edge'='SEDGE'
'in' acts as a link.
(Other definitions for sedge that I've seen before include "Plant of wet ground" , "Wetland grass-like plant" , "Marsh grass" , "Grass-like plant growing in wet places" , "Plant growing in swampy ground" .)