The most common and most likely answer to this crossword clue is the 6 plus 4 letter word FAMILY TREE
I've seen this crossword clue in The Guardian
F
A
M
I
L
Y
T
R
E
E
More clues leading to the result FAMILY TREE
Buddy explains!
The straight part
We know that "A relatively complex document" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a relatively complex document = family tree.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.