Instrument – Twelfth Night character (5)
I believe the answer is:
viola
I believe this clue is a double definition.
'instrument' is the first definition.
(viola is a kind of instrument)
'twelfth night character' is the second definition.
(I've seen this in another clue)
(Other definitions for viola that I've seen before include "whose player takes a bow" , "Pansy-like plant" , "Sebastian's twin (Twelfth Night)" , "Shakespearean character" , "big fiddle!" .)