Ukulele accessory (4)
I believe the answer is:
capo
'ukulele accessory' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for capo that I've seen before include "Mafia boss" , "Head honcho in Cosa Nostra" , "crime boss" , "Mafia leader" , "Gangster boss" .)