The most common and most likely answer to this crossword clue is the 9 letter word DEFENDERS
I've seen this crossword clue in The Times
D
E
F
E
N
D
E
R
S
Buddy explains!
The straight part
We know that "Group under attack" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case group under attack = defenders.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 DEFENDERS mean?
DEFENDERS noun- Someone who defends people or property
- One of the players whose primary task is to prevent the opposition from scoring
- A fighter who seeks to repel an attack
- A lawyer who represents defendants, especially a public defender; a defense attorney (US) or defence counsel (UK)
- A defendant in a civil action