Who's a pretty boy? A Mafia boss is! (6)
I believe the answer is:
'who's a pretty boy?' is the definition.
I can't tell whether this defines the answer.
'a mafia boss is' is the wordplay.
'mafia boss' becomes 'don' (term for a mafia leader).
Can you help me to learn more?
(Other definitions for adonis that I've seen before include "A very handsome legendary Greek" , "striking youth" , "Beautiful (Greek) man" , "Greek idol" , "lovely chap" .)