Old French artist, largely upset (4)
I believe the answer is:
aged
'old' is the definition.
(similar in meaning)
'french artist largely upset' is the wordplay.
'french artist' becomes 'degas' (I've seen this in another clue).
'largely' means to remove the last letter.
'upset' says the letters should be written in reverse.
'degas' with its last letter taken off is 'dega'.
'dega' back-to-front is 'AGED'.
(Other definitions for aged that I've seen before include "Ripened, matured" , "Got older" , "not a modern copy" , "Getting on" , "Time-worn" .)