Warning sign (4)
'warning sign' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for red flag that I've seen before include "The ... ..., anthem of the British Labour Party" , "Emblem of socialist revolution or danger" , "A number of revolutionaries" , "Warning signal" , "Emblem of socialism" .)
'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 "Gloomy sign" , "Portent, prophetic indicator" , "Forewarning, harbinger" , "Portent of things to come" , "Straw in the wind" .)