Wetland plant (4)
I believe the answer is:
reed
'wetland plant' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for reed that I've seen before include "A blade of march grass" , "Instrument; tall grass" , "'It vibrates in a clarinet, say (4)'" , "Marsh or water plant" , "It grows in water or marshy land" .)