Plant providing small margin (5)
I believe the answer is:
sedge
'plant' is the definition.
(sedge is a plant)
'small margin' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'margin' becomes 'edge' (synonyms).
's'+'edge'='SEDGE'
'providing' acts as a link.
(Other definitions for sedge that I've seen before include "Coarse, grasslike plant" , "Plant of wet ground" , "Waterside plant" , "Marsh-dweller" , "fly by rivers" .)