Renaissance polymath upsetting old doctor with article on Xmas (8)
I believe the answer is:
leonardo
'renaissance polymath' is the definition.
(Leonardo da Vinci)
I can't explain the rest of the clue.
(Other definitions for leonardo that I've seen before include "Italian boy" , "US actor, - DiCaprio" , "Da Vinci's name" , "Da Vinci and Di Caprio" , "The Mona Lisa was painted by . . . . . . . . da Vinci" .)