The most common and most likely answer to this crossword clue is the 7 letter word GESTATE
I've seen this crossword clue in The Sunday Telegraph
G
E
S
T
A
T
E
Buddy explains!
The straight part
We know that "to develop over long period" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to develop over long period = gestate.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 GESTATE mean?
GESTATE verb- To carry offspring in the uterus from conception to delivery.
- (by analogy) To develop an idea.