Six-stringed instrument (6)
I believe the answer is:
guitar
'six-stringed instrument' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for guitar that I've seen before include "One played with plectrum" , "Axe" , "Popular stringed musical instrument" , "Stringed instrument often played by pop groups" , "Plucked instrument" .)