Crossword Genius

Side (with)

Ross

I believe the answer is:

ally

(Other definitions for ally that I've seen before include "State formally cooperating with another" , "Band together" , "Someone with whom one is in league" , "Friend in time of need" , "mate" .)

I've seen this clue in The New York 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.