Unclip, stop, point your unclipped leg to the side, fall over on that leg, absorb impact with knee, discombobulate, stand there with one foot and the bike leaning to the side and wait for a chance to set off again. If you want to get going again, push off with the leg on the ground and start moving.

Or alterantively if you have long feet and low BB heigh, stand on your toes. That's usually enough for short stops.

I'll usually stay clipped in with both feet if there's a convenient light pole or one of those neat steel beams meant for cyclists to hold onto while waiting for the light to change.