Plant's small advantage (5)
I believe the answer is:
sedge
'plant's' is the definition.
The answer is an example.
'small advantage' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'advantage' becomes 'edge' (edge is a kind of advantage).
's'+'edge'='SEDGE'
(Other definitions for sedge that I've seen before include "Plant of wet ground" , "Marsh-dweller" , "water-loving vegetation" , "Plant growing in swampy ground" , "Herons" .)