When you put a bike into 1st gear that has a dog engagement window you will get a noticeable clunk. This is normal. Its because you have a rotating mass slamming into a stationary mass. If you don't want this to happen then you either have to shift into first gear before coming to a complete stop or start the bike in 1st with the clutch held in. But if you ever go from N to 1, then you taking the rotating gear cluster that is connected though the clutch to the engine and slamming it into the stationary gear set that is attached to the chain and wheel. It will make some noise, but it is totally normal.