The most common and most likely answer to this crossword clue is the 10 letter word PERCEPTIVE
I've seen this crossword clue in The Guardian
P
E
R
C
E
P
T
I
V
E
More clues leading to the result PERCEPTIVE
Buddy explains!

The straight part
We know that "alert" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case alert = perceptive.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 PERCEPTIVE mean?
PERCEPTIVE adjective- Having or showing keenness of perception, insight, understanding, or intuition.