Spies on the Greek Underworld briefly by protecting the viewers? (8)
I believe the answer is:
eyeshade
'spies on the greek' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'underworld briefly by protecting the viewers?' is the wordplay.
'underworld' becomes 'hades' ('Hades' can be a synonym of 'underworld').
'briefly' means to remove the last letter.
'by' is a charade indicator (letters next to each other).
'protecting the viewers?' becomes 'eyes' (I can't explain this - if you can you should believe this answer much more).
'hades' with its final letter taken off is 'hade'.
'hade' after 'eyes' is 'EYESHADE'.
Can you help me to learn more?
(Other definitions for eyeshade that I've seen before include "Translucent visor" , "something like CAP" , "Visor worn by card-players" , "Protective visor" , "Sun visor" .)