Cousin of a bassoon (4)
I believe the answer is:
oboe
'cousin of a bassoon' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for oboe that I've seen before include "Double-reeded woodwind instrument" , "Reed instrument" , "source of notes" , "Source of music" , "Slender double-reed instrument" .)