Region of Pluto in darkness with sun going down (5)
I believe the answer is:
hades
'region of pluto' is the definition.
(in Greek myth, Pluto ruled the underworld Hades)
'in darkness with sun going down' is the wordplay.
'in' is an insertion indicator.
'darkness with' becomes 'hes' (I can't justify this - if you can you should believe this answer much more).
'sun' becomes 's'.
'going' indicates named letters should be taken away.
'down' becomes 'sad' (I've seen this before).
'sad' with 's' taken away is 'ad'.
'hes' enclosing 'ad' is 'HADES'.
(Other definitions for hades that I've seen before include "Pluto" , "Greek underworld with spirits of the dead" , "Inferno" , "Underworld, hell" , "Classic hell in the shade" .)