The most common and most likely answer to this crossword clue is the 8 letter word SUBADULT
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U
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Buddy explains!

The straight part
We know that "not quite fully developed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not quite fully developed = subadult.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 SUBADULT mean?
SUBADULT noun- A person who, or animal that, is not yet an adult.
- Not yet adult.