Underworld spooks beheaded (5)
I believe the answer is:
hades
'underworld' is the definition.
(Hades can mean the underworld in Greek myth)
'spooks beheaded' is the wordplay.
'spooks' becomes 'shades' ('shade' can be a synonym of 'spook').
'beheaded' means to remove the first letter (the head of the word is removed).
'shades' with its first letter removed is 'HADES'.
(Other definitions for hades that I've seen before include "Mythological abode of the dead" , "Pluto" , "Inferno" , "A place of some darkness" , "It's hell in the Greek shade" .)