Underworld keeps around half of dead (5)
I believe the answer is:
hades
'underworld' is the definition.
(synonyms)
'keeps around half of dead' is the wordplay.
'keeps' becomes 'has' (I've seen this in another clue).
'around' is an insertion indicator.
'half of' suggests halving the letters.
'dead' with half the letters taken is 'de'.
'has' enclosing 'de' is 'HADES'.
(Other definitions for hades that I've seen before include "In Greek myth, the Underworld" , "gloomy place" , "Classical underworld" , "Home of the souls of the dead" , "Underworld, hell" .)