Clarinetist's buy (4)
I believe the answer is:
reed
'clarinetist's buy' 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 stiff marsh or water grass" , "Marsh plant with tall straight stem" , "Vibrating part of some instruments" , "It grows in water or marshy land" .)