The most common and most likely answer to this crossword clue is the 6 letter word GROWER
G
R
O
W
E
R
More clues leading to the result GROWER
Buddy explains!

The straight part
We know that "One developing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one developing = grower.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 GROWER mean?
GROWER noun- A farmer; one who grows things.
- Something that grows.
- Someone or something who becomes more likeable over time
- A man whose penis does not show its full size until it is erect.