Marsh stalk (4)
I believe the answer is:
reed
'marsh stalk' 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 "Marsh plant with tall hollow stem" , "Tall stiff marsh or water grass" , "Film director" , "Plant of marshy land" , "Tall woody grass" .)