The most common and most likely answer to this crossword clue is the 14 letter word UNDERDEVELOPED
I've seen this crossword clue in The Times
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More clues leading to the result UNDERDEVELOPED
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The straight part
We know that "Showing immaturity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case showing immaturity = underdeveloped.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 UNDERDEVELOPED mean?
UNDERDEVELOPED verb- To develop insufficiently.
- Immature and not fully developed
- Having a low level of economic productivity and technological sophistication
- Of a film that has had insufficient development before being fixed