Opera in which dog devours priest (7)
I believe the answer is:
fidelio
'opera' is the definition.
(I've seen this before)
'dog devours priest' is the wordplay.
'dog' becomes 'fido' (I've seen this before).
'devours' means one lot of letters goes inside another (inserted letters are devoured).
'priest' becomes 'eli' (biblical figure).
'fido' placed around 'eli' is 'FIDELIO'.
'in which' is the link.
(Other definitions for fidelio that I've seen before include "Noted work" , "Noted performance" , "18 OPERA]" , "Beethoven opera; I.e. if old (anag.)" , "Beethoven's only opera" .)