Mouthpiece for 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)
'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 "Medium of information - most churches have one" , "Large musical instrument often heard in church" , "Instrument with stops" , "daily?" , "Agency" .)