Warning sign (4)
I believe the answer is:
omen
'warning sign' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for omen that I've seen before include "Portent of things to come" , "Sign of something about to happen" , "Sign of things to come" , "Forewarning, harbinger" , "Sign of likely future event" .)