Marsh grasses (5)
I believe the answer is:
reeds
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">marsh grasses</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for reeds that I've seen before include "Tall slender grasses of wet areas" , "Woodwind instruments" , "Plants providing thatching material" , "Tall stiff marsh grasses" , "Water plants" .)