Baby beginning to eat solid (4)
I believe the answer is:
cube
'solid' is the definition.
(I know that cube is a type of regular polyhedron)
'baby beginning to eat' is the wordplay.
'baby' becomes 'cub' (I can't explain this - if you can you should give a lot more credence to this answer).
'beginning to' says to take the initial letters.
The first letter of 'eat' is 'e'.
'cub'+'e'='CUBE'
Can you help me to learn more?
(Other definitions for cube that I've seen before include "Hexahedron with six plane faces" , "Square-faced solid" , "Form of chiller" , "Hexagonal ice" , "Cut into solid squares" .)