The most common and most likely answer to this crossword clue is the 7 plus 7 letter word BARRAGE BALLOON
B
A
R
R
A
G
E
B
A
L
L
O
O
N
Buddy explains!
The straight part
We know that "Air defence" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case air defence = barrage balloon.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.