Terrible gangster (8)
I believe the answer is:
criminal
This clue is a double definition.
'terrible' is the first definition.
(thesaurus)
'gangster' is the second definition.
(thesaurus)
(Other definitions for criminal that I've seen before include "Being sought by police" , "Violating the law" , "Lawless, felonious" , "Mafioso, possibly" , "A bad hat" .)