The most common and most likely answer to this crossword clue is the 6 letter word GROWTH
I've seen this crossword clue in The Daily Telegraph
G
R
O
W
T
H
Buddy explains!

The straight part
We know that "development" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case development = growth.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 GROWTH mean?
GROWTH noun- An increase in size, number, value, or strength.
- The act of growing, getting bigger or higher.
- Something that grows or has grown.
- An abnormal mass such as a tumor.