Head of Mafia to request disguise (4)
I believe the answer is:
mask
'disguise' is the definition.
(masking is a kind of disguising)
'head of mafia to request' is the wordplay.
'head of' indicates taking the first letters.
'to request' becomes 'ask' ('ask' can be a synonym of 'request').
The initial letter of 'mafia' is 'm'.
'm'+'ask'='MASK'
(Other definitions for mask that I've seen before include "One to hide face" , "Facial disguise" , "Face-covering" , "Visor; disguise" , "Domino" .)