Glass instrument (5)
I believe the answer is:
flute
This clue is a double definition.
'glass' is the first definition.
(I know that flute is a type of wine glass)
'instrument?' is the second definition.
(I know that flute is or was a flute)
(Other definitions for flute that I've seen before include "Narrow wineglass" , "Musical instrument - wineglass" , "Wind instrument" , "Blown" , "High-pitched woodwind instrument" .)