Bassoon insert (4)
I believe the answer is:
reed
'bassoon insert' 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 "Thin strip that vibrates to produce a note" , "Vibrating part of some instruments" , "Tall grass of wet areas" , "Vibrator in the mouthpiece of a wind instrument" , "Stalk" .)