Shape youngster with energy (4)
I believe the answer is:
cube
'shape' is the definition.
(I know that cube is a type of solid)
'youngster with energy' is the wordplay.
'youngster' becomes 'cub' (young bear).
'with' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mcu).
'cub'+'e'='CUBE'
(Other definitions for cube that I've seen before include "Regular 3-D shape" , "product of three factors" , "Three dimetional shape" , "Hexahedron with six plane faces" , "Square-sided solid" .)