Oboe accessory (4)
I believe the answer is:
reed
'oboe accessory' 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 "oboe part" , "'It vibrates in a clarinet, say (4)'" , "Type of grass" , "Plant of marshy land" , "Tall grass at water's edge" .)