Unaware that one has the wrong key? (4-4)
I believe the answer is:
tone-deaf
'unaware' is the definition.
Both the definition and answer are adjectives. Maybe there's an association between them I don't understand?
'one has the wrong key?' is the wordplay.
'has' is a charade indicator (letters next to each other).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'wrong key?' becomes 'deaf' (I can't explain this - if you can you should give a lot more credence to this answer).
'one' put after 't' is 'tone'.
'tone'+'deaf'='TONE-DEAF'
'that' is the link.
Can you help me to learn more?
(Other definitions for tone-deaf that I've seen before include "Unable to distinguish variations in musical pitch" , "Musically challenged" , "Unable to perceive differences of musical pitch" , "Unable to perceive musical pitch accurately" , "To deafen? (anag)" .)