Greeting Underworld, Orpheus originally (5)
I believe the answer is:
hello
'greeting' is the definition.
(hello is a kind of greeting)
'underworld orpheus originally' is the wordplay.
'underworld' becomes 'hell' (synonyms).
'originally' says to take the initial letters.
The initial letter of 'orpheus' is 'o'.
'hell'+'o'='HELLO'
(Other definitions for hello that I've seen before include "Meeting greeting" , "Introductory greeting" , "Conversation starter" , "Common greeting as on phone" , "Welcome" .)