Marsh grasses (5)
I believe the answer is:
reeds
'marsh grasses' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)