Upcoming artist mostly past his prime (4)
I believe the answer is:
aged
'past his prime' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I can't see?
'upcoming artist mostly' is the wordplay.
'upcoming' is a reversal indicator (in a down clue coming up is backwards).
'artist' becomes 'Degas' (painter Edgar Degas).
'mostly' means to remove the last letter (most but not all of the word).
'degas' with its last letter taken off is 'dega'.
'dega' written backwards gives 'AGED'.
Can you help me to learn more?
(Other definitions for aged that I've seen before include "Got older" , "Long in the tooth" , "Advanced in years" , "Matured like wine in wood" , "Like Uncle Arly" .)