Gulf state protecting mafia head shoots official investigator (9)
I believe the answer is:
ombudsman
'official investigator' is the definition.
(type of public official)
'gulf state protecting mafia head shoots' is the wordplay.
'gulf state' becomes 'oman' (I've seen this in another clue).
'protecting' means one lot of letters goes inside another (inserted letters are held or protected).
'head' says to take the initial letters.
'shoots' becomes 'buds'.
The first letter of 'mafia' is 'm'.
'm'+'buds'='mbuds'
'oman' enclosing 'mbuds' is 'OMBUDSMAN'.
(Other definitions for ombudsman that I've seen before include "Commisioner" , "Official pursuing complaints against government departments" , "Malpractice investigator" , "Official investigator of complaints" , "Government appointee who investigates complaints" .)