Duet with nun terribly out of key (7)
I believe the answer is:
untuned
'out of key' is the definition.
(I've seen this in another clue)
'duet with nun terribly' is the wordplay.
'with' is a charade indicator (letters next to each other).
'terribly' indicates an anagram.
'duet'+'nun'='duetnun'
'duetnun' anagrammed gives 'UNTUNED'.
(Other definitions for untuned that I've seen before include "With no fixed pitch" , "(Of piano) not adjusted" .)