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