That is an incredibly vague question. The answer largely depends on what you can handle easily, and we don't know your experience, chosen carry method, and what you're wanting it to be "lighter" than.
All that being said, the one gun that goes with me everywhere, in all attire, is easily handled, capable, lightweight, and carries a respectable amount of rounds of a respectable caliber, is my Kel-Tec P32. I'm usually carrying it as a backup to another, bigger gun, but the P32 would be a contender for being my personal go-to if I could only have one carry gun. It's around nine ounces fully loaded, less than 3/4 of an inch thick, and holds eight rounds, including the one in the chamber. Its longer trigger pull negates the need for any kind of a safety lever; it's a "pull, point, and press-to-play" defensive pistol in every sense. I often carry it with Kel-Tec's optional ten-round magazine in place, giving me eleven rounds of .32 caliber fight if needed.