The most common and most likely answer to this crossword clue is the 7 letter word RAVELIN
I've seen this crossword clue in The Times
R
A
V
E
L
I
N
Buddy explains!
The straight part
We know that "Part of defence" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of defence = ravelin.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 RAVELIN mean?
RAVELIN noun- An outwork. A fortification outside a castle used to split an attacking force; composed of two faces, forming a salient angle whose gorge resembles a half-moon