The underworld is a noted puzzle (6)
I believe the answer is:
erebus
'the underworld' is the definition.
(erebus greek deity born of chaos)
'noted puzzle' is the wordplay.
'noted' becomes 'e' (I can't justify this - if you can you should believe this answer much more).
'puzzle' becomes 'rebus' (I've seen this before).
'e'+'rebus'='EREBUS'
'is a' is the link.
Can you help me to learn more?
(Other definitions for erebus that I've seen before include "The abode of the dead" , "Dark and gloomy cavern between earth and Hades." , "Volcanic mountain in Antarctica" , "Be sure (anag) - Greek god" , "It's hellish dark" .)