The most common and most likely answer to this crossword clue is the 5 plus 8 letter word POTTY TRAINING
I've seen this crossword clue in Financial Times
P
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Buddy explains!
The straight part
We know that "instruction the young must sit through" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case instruction the young must sit through = potty training.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 POTTY TRAINING mean?
POTTY TRAINING noun- The act of weaning a child off of diapers and teaching them how to use a potty. Not usually distinguished from toilet training although generally children do not use an adult-sized toilet right away.