Shock in underworld after rising Mafia boss cleared out (9)
I believe the answer is:
bombshell
'shock' is the definition.
(bombshell can mean a great shock)
'underworld after rising mafia boss cleared out' is the wordplay.
'underworld' becomes 'hell' (synonyms).
'after' says to put letters next to each other.
'rising' is a reversal indicator (going up in a down clue).
'mafia' becomes 'mob' (informal term for the mafia).
'cleared out' suggests removing the centre.
'boss' with its centre removed is 'bs'.
'mob' back-to-front is 'bom'.
'bom'+'bs'='bombs'
'hell' put after 'bombs' is 'BOMBSHELL'.
'in' is the link.
(Other definitions for bombshell that I've seen before include "A great surprise" , "Thunderbolt, shocking surprise" , "big disappointment" , "Big surprise" , "Sudden disappointment" .)