Grassy plant of wet areas (4)
I believe the answer is:
reed
'grassy plant of wet areas' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for reed that I've seen before include "Tall grass plant that grows in water" , "Blade of tall marsh grass" , "Rush" , "piece for organ" , "Tall water-loving grass" .)