Glenn Miller’s instrument (8)
I believe the answer is:
trombone
'glenn miller's instrument' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for trombone that I've seen before include "Brass instrument with sliding tubes" , "Blow me!" , "Bfrass wind instrument with slide" , "Brass musical instrument with slide" , "source of notes" .)