Cause for concern in a relationship (3,4)

I believe the answer is:

red flag

(Other definitions for red flag that I've seen before include "Banner used to signal danger - or communism" , "A flyer" , "A number of revolutionaries" , "Warning signal" , "Socialist banner?" .)

I've seen this clue in the Universal.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.