The most common and most likely answer to this crossword clue is the 9 letter word VALIDATED
I've seen this crossword clue in The Daily Telegraph
V
A
L
I
D
A
T
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D
More clues leading to the result VALIDATED
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 = validated.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 VALIDATED mean?
VALIDATED verb- To render valid.
- To check or prove the validity of; verify.
- To have its validity successfully proven.
- Declared or made valid.
- Having experienced emotional validation.