The most common and most likely answer to this crossword clue is the 5 letter word FULLY
I've seen this crossword clue in The Sunday Telegraph
F
U
L
L
Y
More clues leading to the result FULLY
Buddy explains!

The straight part
We know that "completely developed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case completely developed = fully.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 FULLY mean?
FULLY adverb- In a full manner; without lack or defect.
- In a full degree; to a full extent.
- As a minimum; at least.