The key to the puzzle is infernally deep (6)
I believe the answer is:
erebus
'infernally deep' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'the key to the puzzle' is the wordplay.
'the key' becomes 'E' (a musical key).
'to the puzzle' becomes 'rebus' (type of visual puzzle).
'e'+'rebus'='EREBUS'
'is' is the link.
Can you help me to learn more?
(Other definitions for erebus that I've seen before include "Greek god of darkness, son of Chaos" , "In place of darkness" , "Dark and gloomy cavern between earth and Hades." , "God of darkness - cavern on the way to Hades - active volcano in Antarctica" , "Mt. -, Antarctic volcano; be sure (anag.)" .)