The most common and most likely answer to this crossword clue is the 8 letter word DESERVES
I've seen this crossword clue in The Daily Telegraph
D
E
S
E
R
V
E
S
Buddy explains!

The straight part
We know that "Justifies" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case justifies = deserves.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 DESERVES mean?
DESERVES verb- To be entitled to, as a result of past actions; to be worthy to have.
- To earn, win.
- To reward, to give in return for service.
- To serve; to treat; to benefit.