Youngster seen by an islander (5)
I believe the answer is:
cuban
'islander' is the definition.
(I've seen this before)
'youngster seen by an' is the wordplay.
'youngster' becomes 'cub' (cub can mean a young person).
'seen by' is a charade indicator (letters next to each other).
'cub'+'an'='CUBAN'
(Other definitions for cuban that I've seen before include "Republican" , "One from Havana" , "from island republic" , "Relating to Castro's isle" , "Nationality of citizen of Havana" .)