Bassoonist's buy (4)
I believe the answer is:
reed
'bassoonist'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 "Type of grass" , "Blade of tall marsh grass" , "Stalk" , "It grows in water or marshy land" , "Marsh-growing grass" .)