Clarinet's relative (4)
I believe the answer is:
oboe
'clarinet's relative' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oboe that I've seen before include "Musical instrument" , "Producer of notes" , "Double-reeded woodwind instrument" , "Instrument used in 1d" , "Double-reed wind instrument" .)