The most common and most likely answer to this crossword clue is the 5 plus 4 plus 4 letter word WATCH ONES BACK
I've seen this crossword clue in The Times
W
A
T
C
H
O
N
E
S
B
A
C
K
Buddy explains!
The straight part
We know that "be alert to attack" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case be alert to attack = watch ones back.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.