A most unusual church organ (7)
I believe the answer is:
stomach
'organ' is the definition.
(stomach is a kind of organ)
'a most unusual church' is the wordplay.
'unusual' is an anagram indicator.
'church' becomes 'ch' (common abbreviation).
'a'+'most'='amost'
'amost' anagrammed gives 'stoma'.
'stoma'+'ch'='STOMACH'
(Other definitions for stomach that I've seen before include "Endure; organ" , "Inside" , "Courage - guts" , "Mac hosts his organ" , "Bear - belly" .)