The most common and most likely answer to this crossword clue is the 9 letter word BACKSTORY
I've seen this crossword clue in The Guardian
B
A
C
K
S
T
O
R
Y
Buddy explains!
The straight part
We know that "what happened before start of film" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what happened before start of film = backstory.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 BACKSTORY mean?
BACKSTORY 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.