The most common and most likely answer to this crossword clue is the 8 letter word VERIFIED
I've seen this crossword clue in The Times
V
E
R
I
F
I
E
D
Buddy explains!

The straight part
We know that "Backed up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case backed up = verified.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 VERIFIED mean?
VERIFIED noun- A user of the Twitter microblogging service whose identity has been confirmed by Twitter.
- Subject to positive verification.
- To substantiate or prove the truth of something
- To confirm or test the truth or accuracy of something
- To affirm something formally, under oath