Youngster first to become an artist (6)
I believe the answer is:
cubist
'an artist' is the definition.
(cubist is a kind of artist)
'youngster first to' is the wordplay.
'youngster' becomes 'cub' (cub can mean a young person).
'first to' becomes 'ist' (I've seen this in other clues).
'cub'+'ist'='CUBIST'
'become' acts as a link.
(Other definitions for cubist that I've seen before include "Geometrical painter" , "Braque, for one" , "Maybe multifaceted" , "modern artist" , "Painter like Picasso" .)