A racket taking in head of mafia organisation (5)
I believe the answer is:
admin
'organisation' is the definition.
(short for administration)
'a racket taking in head of mafia' is the wordplay.
'racket' becomes 'din' (both can mean a loud noise or disturbance).
'taking in' means one lot of letters goes inside another.
'head of' indicates taking the first letters.
The first letter of 'mafia' is 'm'.
'din' enclosing 'm' is 'dmin'.
'a'+'dmin'='ADMIN'
(Other definitions for admin that I've seen before include "Paperwork (informal)" , "Back-office work (abbr)" , "Management function" , "Business organisation" , "Paperwork (abbr)" .)