Youngster close to stone block (4)
I believe the answer is:
cube
'block' is the definition.
(cube is a kind of block)
'youngster close to stone' is the wordplay.
'youngster' becomes 'cub' (I've seen this in other clues).
'close to' says to take the final letters (the 'close' to the word).
The final letter of 'stone' is 'e'.
'cub'+'e'='CUBE'
(Other definitions for cube that I've seen before include "Square block" , "Form of chiller" , "product of three factors" , "Six-sided body" , "Hexahedron with six plane faces" .)