The most common and most likely answer to this crossword clue is the 8 letter word BASTIONS
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More clues leading to the result BASTIONS
Buddy explains!

The straight part
We know that "Defenders" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case defenders = bastions.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 BASTIONS mean?
BASTIONS noun- A projecting part of a rampart or other fortification.
- A well-fortified position; a stronghold or citadel.
- A person, group, or thing, that strongly defends some principle.
- To furnish with a bastion.