Cathedral instrument (5)
I believe the answer is:
organ
'cathedral instrument' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for organ that I've seen before include "Bodily part or large musical instrument" , "Wind and keyboard instrument" , "House journal" , "Musical instrument - pancreas, perhaps" , "Medium of information - most churches have one" .)