Mouthpiece for instrument (5)
I believe the answer is:
Here is my best explanation:
I believe this clue is a double definition.
'mouthpiece' is the first definition.
(I've seen this before)
'instrument' is the second definition.
(organ is a kind of instrument)
'for' acts as a link.
(Other definitions for organ that I've seen before include "Pipe or reed instrument", "daily?", "Stopped instrument", "Soft part of the body which has a specific function", "Eg, heart/liver", "House journal", "Biological functionary", "Large musical instrument often heard in church", "notes issue from this" and "Bodily part or large musical instrument".)