The most common and most likely answer to this crossword clue is the 5 plus 7 letter word LEGAL FICTION
I've seen this crossword clue in The Times
L
E
G
A
L
F
I
C
T
I
O
N
Buddy explains!
The straight part
We know that "that is officially assumed to be true?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that is officially assumed to be true? = legal fiction.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.