Swamp stalk (4)
I believe the answer is:
reed
'swamp stalk' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for reed that I've seen before include "Film director" , "Tall grass of wet areas" , "Plant of marshy land" , "Tall grass plant that grows in water" , "piece for organ" .)