Coarse grass (4)
I believe the answer is:
reed
'coarse grass' 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 "Carol" , "Slender grass of wet areas" , "A blade of march grass" , "ancient arrow" , "One of two in a 25 [OBOE]" .)