Millionaire leaving his estate for good, born during Christmas (5)
I believe the answer is:
nobel
'millionaire leaving his estate for good' is the definition.
(Alfred Nobel's estate funded the Nobel prizes)
'born during christmas' is the wordplay.
'born' becomes 'b'.
'during' means one lot of letters goes inside another.
'christmas' becomes 'Noel' (synonyms).
'b' put inside 'noel' is 'NOBEL'.
(Other definitions for nobel that I've seen before include "Prize giver" , "Source of prizes" , "and 21 Across: Prestigious international award" , "6 24 113 [SCAN-DIN-AVIAN] chemist" , "Alfred --, Swedish founder of international prizes" .)