The most common and most likely answer to this crossword clue is the 8 letter word EVOLVING
I've seen this crossword clue in The Daily Telegraph
E
V
O
L
V
I
N
G
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 = evolving.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 EVOLVING mean?
EVOLVING verb- To move in regular procession through a system.
- To change; transform.
- To come into being; develop.
- Of a population, to change genetic composition over successive generations through the process of evolution.
- To give off (gas, such as oxygen or carbon dioxide during a reaction).
- To cause something to change or transform.