The most common and most likely answer to this crossword clue is the 4 plus 3 letter word LOOK OUT
I've seen this crossword clue in The Daily Telegraph
L
O
O
K
O
U
T
More clues leading to the result LOOK OUT
Buddy explains!
The straight part
We know that "Be vigilant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case be vigilant = look out.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 LOOK OUT mean?
LOOK OUT noun- A vantage point with a view of the surrounding area.
- A session of watching for an approaching enemy, police, etc.
- A person on watch for approaching enemy, police, danger, etc.
- A subject for observation; a prospect or view.
- One's perspective, outlook; hence, one's responsibility. (used with a possessive pronoun or a noun in a possessive form).