Girl needing guide through underworld realm of the dead (5)
I believe the answer is:
hazel
'girl' is the definition.
(girl's name)
'guide through underworld realm of the dead' is the wordplay.
'guide' becomes 'AZ' (A-Z atlas).
'through' means one lot of letters goes inside another (inserted letters go through the word).
'underworld realm of the dead' becomes 'Hel' (underworld in Norse mythology).
'az' inserted within 'hel' is 'HAZEL'.
'needing' acts as a link.
(Other definitions for hazel that I've seen before include "Light brown; nut" , "Cob (nut)" , "Girl's name; colour" , "Nut-bearing shrub" , "Tree; girl's name" .)