Like a guard on duty (5)

I believe the answer is:

alert

(Other definitions for alert that I've seen before include "'Attentive, watchful (5)'" , "Watchful, quick-thinking" , "On the ball" , "Prompt" , "Wide-awake and watchful" .)

I've seen this clue in the LA Times.

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.