One introducing policeman before arresting Mafia head (7)
I believe the answer is:
compere
'one introducing' is the definition.
(someone who introduces people on stage)
'policeman before arresting mafia head' is the wordplay.
'policeman' becomes 'cop' (slang term).
'before' becomes 'ere' (poetic term for 'before').
'arresting' indicates putting letters inside.
'head' says to take the initial letters.
The initial letter of 'mafia' is 'm'.
'cop'+'ere'='copere'
'copere' going around 'm' is 'COMPERE'.
(Other definitions for compere that I've seen before include "host show" , "Master of ceremonies" , "M.C." , "(Male) show introducer" , "TV presenter?" .)