The repair method depends on the age and severity of the crack. If it is old and oil soaked, or extends more than one or two inches into the pistol grip, then YES, breaking it cleanly and degreasing is required. Clean the fresh break using a degreaser like trisodium phosphate and hot water, followed with clear hot water and acetone. Dry at least overnight. The best adhesive is Brownell's Accraglas epoxy resin (not the gel). Clamp together using large rubber bands or padded C clamp.
If the crack is less than two inches long, fresh and free of oil, you simply dribble Accrasglas resin onto the crack, and push it in using a toothpick. Clamp and hold overnight as before.
After the expox dries, use a sharp knife to scrape off the excess resin, sand, and refinish.