He knows when you've been bad or good (5)

I believe the answer is:

santa

(Other definitions for santa that I've seen before include "A children's favourite may" , "patron at Christmas" , "St Nicholas" , "Christmas visitor" , "visitor bringing gifts" .)

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.