Grasslike plant primarily straggling border (5)
I believe the answer is:
sedge
'grasslike plant' is the definition.
(I've seen this in another clue)
'primarily straggling border' is the wordplay.
'primarily' says to take the initial letters.
'border' becomes 'edge' (synonyms).
The initial letter of 'straggling' is 's'.
's'+'edge'='SEDGE'
(Other definitions for sedge that I've seen before include "plant with triangular stems" , "Plant like grass" , "Herons" , "Plant of wet ground" , "Grass-like plant growing in marshes" .)