The most common and most likely answer to this crossword clue is the 8 letter word ALERTING
I've seen this crossword clue in The Guardian
A
L
E
R
T
I
N
G
More clues leading to the result ALERTING
Buddy explains!

The straight part
We know that "Warning" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case warning = alerting.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 ALERTING mean?
ALERTING verb- To give warning to.