Youngster with European dice (4)
I believe the answer is:
cube
'dice' is the definition.
(synonyms)
'youngster with european' is the wordplay.
'youngster' becomes 'cub' (I've seen this before).
'with' says to put letters next to each other.
'european' becomes 'E' (abbreviation e.g. EU).
'cub'+'e'='CUBE'
(Other definitions for cube that I've seen before include "Solid body with six equal square sides" , "Three-dimensional figure" , "Solid body with six equal square faces" , "Hexahedron with six plane faces" , "Square-sided solid" .)