The most common and most likely answer to this crossword clue is the 6 plus 8 letter word TOILET TRAINING
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Buddy explains!
The straight part
We know that "it’s part of growing up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s part of growing up = toilet 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 TOILET TRAINING mean?
TOILET TRAINING verb- To train (a child) to use the toilet.
- To train (a domestic animal) to defecate and urinate in certain designated places.