Part of a string quartet (5)
I believe the answer is:
viola
'part of a string quartet' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for viola that I've seen before include "Bowed instrument slightly larger than a violin" , "Cesario when in drag" , "String quartet member" , "One played" , "Stringed instrument tuned lower than a violin" .)