One giving Mafia boss gold (5)
I believe the answer is:
donor
'one giving' is the definition.
(someone who donates)
'mafia boss gold' is the wordplay.
'mafia boss' becomes 'don' (term for a mafia leader).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'don'+'or'='DONOR'
(Other definitions for donor that I've seen before include "Sponsor" , "A giver, perhaps of blood" , "Generous type" , "blessed person, relatively" , "Benefactor, giver" .)