Underworld organisation's original address (5)
I believe the answer is:
hello
'address' is the definition.
(thesaurus)
'underworld organisation's original' is the wordplay.
'underworld' becomes 'hell' (synonyms).
'original' suggests taking the first letters (the origin of the word).
The initial letter of 'organisation' is 'o'.
'hell'+'o'='HELLO'
(Other definitions for hello that I've seen before include "royals dominate its pages?" , "opener?" , "I say" , "Common greeting as on phone" , "A salutation" .)