Mitch Miller's instrument (4)
I believe the answer is:
oboe
'mitch miller's instrument' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oboe that I've seen before include "Source of music" , "issuer of notes" , "Double-reed woodwind" , "Woodwind instrument of treble pitch" , "Double-reed wind instrument" .)