Second border, it's like grass (5)
I believe the answer is:
sedge
'it's like grass' is the definition.
(grasslike plant)
'second border' is the wordplay.
'second' becomes 's' (common abbreviation).
'border' becomes 'edge' (synonyms).
's'+'edge'='SEDGE'
(Other definitions for sedge that I've seen before include "Marsh-dweller" , "Plant like grass" , "Grasslike marsh plant" , "Grass-like plant growing in marshes" , "Coarse grassy plant" .)