Small border - it's like grass (5)
I believe the answer is:
sedge
'it's like grass' is the definition.
(grasslike plant)
'small border' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'border' becomes 'edge' (synonyms).
's'+'edge'='SEDGE'
(Other definitions for sedge that I've seen before include "Grass-like plant growing in marshes" , "Grasslike marsh plant" , "plant with triangular stems" , "Plant of wet ground" , "Waterside plant" .)