Sunglasses not first in the underworld (5)
I believe the answer is:
hades
'underworld' is the definition.
(Hades can mean the underworld in Greek myth)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for hades that I've seen before include "The nether world, hell" , "Infernal situation" , "A place of some darkness" , "It's classical hell in the shade" , "Home of the souls of the dead" .)