Youngster with energy gets into shape (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' (cub can mean a young person).
'with' says to put letters next to each other.
'energy' becomes 'e' (physics symbol as in E=mcu).
'cub'+'e'='CUBE'
'gets into' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for cube that I've seen before include "Solid body with six equal square sides" , "product of three factors" , "nine?" , "Three-dimensional square" , "Cut into solid squares" .)