Hat on old Mafia boss (4)
I believe the answer is:
capo
'mafia boss' is the definition.
(I've seen this before)
'hat on old' is the wordplay.
'hat' becomes 'cap' (I've seen this before).
'on' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'cap'+'o'='CAPO'
(Other definitions for capo that I've seen before include "Mafia leader" , "Head honcho in Cosa Nostra" , "crime boss" , "Movable bridge" , "Mafia chief?" .)