Clarinet, for one (4)
I believe the answer is:
reed
'clarinet for one' is the definition.
(clarinet is a kind of reed)
This is all the clue.
(Other definitions for reed that I've seen before include "Grass of marshy land" , "Marsh plant with straight hollow stem" , "Bilk had one" , "A blade of march grass" , "Oliver, say" .)