The most common and most likely answer to this crossword clue is the 8 letter word BULWARKS
I've seen this crossword clue in Financial Times
B
U
L
W
A
R
K
S
More clues leading to the result BULWARKS
Buddy explains!

The straight part
We know that "defences" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case defences = bulwarks.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 BULWARKS mean?
BULWARKS noun- A defensive wall or rampart.
- A defense or safeguard.
- A breakwater.
- The planking or plating along the sides of a nautical vessel above her gunwale that reduces the likelihood of seas washing over the gunwales and people being washed overboard.
- Any means of defence or security.
- To fortify something with a wall or rampart.
- To provide protection of defense for something.