The most common and most likely answer to this crossword clue is the 10 letter word WARRANTING
I've seen this crossword clue in The Daily Telegraph
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Buddy explains!

The straight part
We know that "Justifying" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case justifying = warranting.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 WARRANTING mean?
WARRANTING verb- To protect, keep safe (from danger).
- To give (someone) an assurance or guarantee (of something); also, with a double object: to guarantee (someone something).
- To guarantee (something) to be (of a specified quality, value, etc.).
- To guarantee as being true; (colloquially) to believe strongly.
- To authorize; to give (someone) sanction or warrant (to do something).
- To justify; to give grounds for.
- A guarantee.