Strummed instrument (6)
'strummed instrument' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for zither that I've seen before include "Instrument played on the knees" , "music producer" , "Stringed instrument laid flat to play" , "Plucked musical instrument" , "Harp-like instrument" .)
(Other definitions for guitar that I've seen before include "One played with plectrum" , "Fretted string instrument" , "Talk under one's breath" , "It's plucked or strummed" , "Plucked instrument" .)