The most common and most likely answer to this crossword clue is the 5 plus 4 letter word LAGER LOUT
I've seen this crossword clue in The Daily Telegraph
L
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More clues leading to the result LAGER LOUT
- Drunk upset majestic group outside university
- . . . . rogue, all excited with time for such a youth
- Convict taking circuitous route to welcome large young man, violent when drunk
- Drunken rowdy lecturer dismissed on sovereign’s return
- Tall rogue pummelled offensive drunk
- Convict to rule over boozer
- Unruly youth‘s beer left on display
- Criminal, rude lot misbehaving with day gone — and boorish drinker?
- Drunk tall rogue
- Boozy yob's unexpectedly got allure
Buddy explains!

The straight part
We know that "Unruly youth" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unruly youth = lager lout.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.