Youngster close to three in block (4)
I believe the answer is:
cube
'block' is the definition.
(I know that block can be written as cube)
'youngster close to three' is the wordplay.
'youngster' becomes 'cub' (I've seen this before).
'close to' indicates one should take the final letters (the 'close' to the word).
The final letter of 'three' is 'e'.
'cub'+'e'='CUBE'
'in' is the link.
(Other definitions for cube that I've seen before include "Cut into solid squares" , "Six-sided body" , "Hexagonal ice" , "Lump of sugar" , "Three dimetional shape" .)