Nothing behind underworld greeting (5)
I believe the answer is:
hello
'greeting' is the definition.
(I know that Hello is a type of greeting)
'nothing behind underworld' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'behind' says to put letters next to each other.
'underworld' becomes 'hell' (synonyms).
'o' after 'hell' is 'HELLO'.
(Other definitions for hello that I've seen before include "Welcome" , "Meeting greeting" , "glossy magazine" , "opener?" , "Expression of greeting" .)