Instrument played sitting down (5)
I believe the answer is:
cello
'instrument played sitting down' 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 "10 [INSTRUMENT]" , "Instrument in the violin family" , "Musical instrument like large violin held between knees" , "Bowed musical instrument" , "Tortelier's instrument" .)