The most common and most likely answer to this crossword clue is the 7 letter word TWEENIE
I've seen this crossword clue in The Guardian
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More clues leading to the result TWEENIE
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 = tweenie.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 TWEENIE mean?
TWEENIE noun- A child who is not quite old enough to be a teenager; a tweenager.
- A shot that is hit between the legs; a tweener.
- A 650-watt fresnel motion picture lighting instrument first manufactured by the Mole-Richardson Company.