Shut up about a piece from Toby Young (8)
I believe the answer is:
immature
'young' is the definition.
(I know that young can be written as immature)
'shut up about a piece from toby' is the wordplay.
'shut up' becomes 'immure' (I've seen this in other clues).
'about' is an insertion indicator.
'a piece from toby' becomes 'at' (I can't explain this - if you can you should give a lot more credence to this answer).
'immure' enclosing 'at' is 'IMMATURE'.
Can you help me to learn more?
(Other definitions for immature that I've seen before include "behaviour of child" , "Not yet adult or fully developed" , "Lacking in development" , "Undeveloped - unripe" , "Not yet adult or fully grown" .)