The most common and most likely answer to this crossword clue is the 5 plus 6 letter word STORY EDITOR
I've seen this crossword clue in The Guardian
S
T
O
R
Y
E
D
I
T
O
R
Buddy explains!
The straight part
We know that "one employed to change film script" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one employed to change film script = story editor.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.