Bluegrass instrument (5)
I believe the answer is:
banjo
'bluegrass instrument' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for banjo that I've seen before include "Five-stringed musical instrument" , "Stringed instrument of the guitar family" , "Stringed instrument with long neck" , "Stringed instrument beloved of old minstrel shows" , "Formby's instrument" .)