Grasslike marsh plant (5)
I believe the answer is:
sedge
'grasslike marsh plant' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sedge that I've seen before include "Grass-like plant growing in wet places" , "plant with triangular stems" , "Plant growing in swampy ground" , "Herons" , "Plant like grass" .)