The most common and most likely answer to this crossword clue is the 9 letter word SCHOOLBOY
I've seen this crossword clue in The Guardian
S
C
H
O
O
L
B
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Y
More clues leading to the result SCHOOLBOY
Buddy explains!

The straight part
We know that "Youngster" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case youngster = schoolboy.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SCHOOLBOY mean?
SCHOOLBOY noun- A boy attending school.
- Characteristic of a schoolboy, especially being cheeky, clumsy and socially unaware.