The most common and most likely answer to this crossword clue is the 4 plus 5 letter word BACK STORY
I've seen this crossword clue in The Times
B
A
C
K
S
T
O
R
Y
More clues leading to the result BACK STORY
Buddy explains!
The straight part
We know that "What has occurred before" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what has occurred before = back story.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 BACK STORY mean?
BACK STORY noun- The previous life and experiences of a person, especially a character in a dramatic work, but also a real-life person.
- A fictitious account of a person's history designed to conceal their actual history and motives.
- A prequel.