Large instrument's mouthpiece (5)
I believe the answer is:
organ
This clue is a double definition.
'large instrument's' is the first definition.
(I've seen this before)
'mouthpiece' is the second definition.
(I've seen this before)
(Other definitions for organ that I've seen before include "Eg, heart, liver" , "Medium; body part" , "What makes use of full stops" , "Part with special function" , "stops on one" .)