The most common and most likely answer to this crossword clue is the 13 letter word JUSTIFICATION
I've seen this crossword clue in The Times
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More clues leading to the result JUSTIFICATION
Buddy explains!

The straight part
We know that "Defence" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case defence = justification.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 JUSTIFICATION mean?
JUSTIFICATION noun- A reason, explanation, or excuse which provides convincing, morally acceptable support for behavior or for a belief or occurrence.
- The alignment of text to the left margin (left justification), the right margin (right justification), or both margins (full justification).