Crossword puzzle clue "sign of a challenge"

Sign of a challenge

Good sign or bad sign

Warning sign at a train station's food court? (NOTE: The original sign, without my additions, actually exists. It's in Grand Central Terminal in NYC, on the lower level — the food court level. The sig

Sign of danger — sign of socialism

Pre-sign sign

Travel High School Challenge sponsor

Up to the challenge

Breeze through a challenge