A 1 arm snatch is basically bringing the DB off the floor, in one hand, to up over your head. You start with the DB on the floor, get down into a beiginning DL stance, grab the bell, stand erect while pulling the bell over your head. In the clean and press, same start, just bring the ell up to your shoulders, then press it over head. Swings, stand erect grabbing the bell in both hands, and keep the arms straight throughout the movementbend down into the DL position , and swing it up in front of you while whipping the hips into it while you stand erect. More descriptions and pictures here
Just look the lift up in the index and click on it.
The were written and shown using kettlebells, but DB's work just fine.