Mouthpiece for musical instrument (5)
I believe the answer is:
organ
I believe this clue is a double definition.
'mouthpiece' is the first definition.
(I've seen this before)
'musical instrument' is the second definition.
(organ is a kind of musical instrument)
'for' is the link.
(Other definitions for organ that I've seen before include "periodical" , "Air-powered instrument" , "notes issue from this" , "Eg, heart/liver" , "Heart or lung, say" .)