Bog plant in small border (5)
I believe the answer is:
sedge
'bog plant' is the definition.
(sedge is a kind of bog plant)
'small border' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'border' becomes 'edge' (synonyms).
's'+'edge'='SEDGE'
'in' is the link.
(Other definitions for sedge that I've seen before include "Grass-like plant growing in wet places" , "Plant growing in swampy ground" , "fly by rivers" , "Coarse, grasslike plant" , "water-loving vegetation" .)