Danger sign (3,4)
I believe the answer is:
red flag
'danger sign' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for red flag that I've seen before include "Danger signal - keep it flying high!" , "The ... ..., anthem of the British Labour Party" , "Sign of danger - sign of socialism" , "left-wing message?" , "Warning signal" .)