Violin's big brother (5)
I believe the answer is:
cello
'violin's big brother' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cello that I've seen before include "Musical instrument like large violin held between knees" , "big fiddle?" , "String quartet instrument" , "It's bigger than a viola, but smaller than a double bass" , "Large stringed instrument" .)