Artists first impressed by youngsters (7)
I believe the answer is:
cubists
'artists' is the definition.
(cubist is a kind of artist)
'first impressed by youngsters' is the wordplay.
'first' becomes 'ist' (ist resembles 1st).
'impressed by' is an insertion indicator.
'youngsters' becomes 'cubs' (cub can mean a baby or child).
'ist' going inside 'cubs' is 'CUBISTS'.
(Other definitions for cubists that I've seen before include "group of artists" , "Disciples of Picasso, perhaps" , "Painters" .)