Grasslike marsh plant (5)
I believe the answer is:
sedge
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">grasslike marsh plant</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)