The most common and most likely answer to this crossword clue is the 8 plus 10 letter word PLANNING PERMISSION
I've seen this crossword clue in The Times
P
L
A
N
N
I
N
G
P
E
R
M
I
S
S
I
O
N
Buddy explains!
The straight part
We know that "OK to develop" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ok to develop = planning permission.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.