Plants found by southern borders (6)
I believe the answer is:
sedges
'plants' is the definition.
The answer is an example.
'by southern borders' is the wordplay.
'by' means one lot of letters go next to another.
'southern' becomes 'S' (abbreviation).
'borders' becomes 'edges' ('edge' can be a synonym of 'border').
's'+'edges' is 'SEDGES'.
'found' is the link.
(Other definitions for sedges that I've seen before include "Rush-like plants growing in wet places" , "Flies" .)