The most common and most likely answer to this crossword clue is the 9 letter word PUBESCENT
I've seen this crossword clue in Financial Times
P
U
B
E
S
C
E
N
T
Buddy explains!

The straight part
We know that "Developing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case developing = pubescent.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 PUBESCENT mean?
PUBESCENT noun- An individual who is going through puberty.
- At or just after the age of puberty.
- Covered with down or fine hairs.