Elderly French artist, for the most part upset (4)
I believe the answer is:
aged
'elderly' is the definition.
(synonyms)
'french artist for the most part upset' is the wordplay.
'french artist' becomes 'degas' (Edgar Degas, 1834-1917).
'for the most part' means to remove the last letter (most but not all of the letters).
'upset' is a reversal indicator.
'degas' with its final letter taken away is 'dega'.
'dega' reversed gives 'AGED'.
(Other definitions for aged that I've seen before include "Grew old" , "Antiquated" , "Getting on" , "Geriatric" , "Like Uncle Arly" .)