The most common and most likely answer to this crossword clue is the 8 letter word DISARMED
I've seen this crossword clue in The Sunday Telegraph
D
I
S
A
R
M
E
D
More clues leading to the result DISARMED
Buddy explains!

The straight part
We know that "Defenceless" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case defenceless = disarmed.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 DISARMED mean?
DISARMED verb- To deprive of weapons; to deprive of the means of attack or defense; to render defenseless.
- To deprive of the means or the disposition to harm; to render harmless or innocuous
- To lay down arms; to stand down.
- To reduce one's own military forces.
- To disable the security systems on.