Motorcycle Repair: clutch, suitable piece, clutch

Question
i am trying to replace the clutch on my bike and there is some kind of nut that is stopping it coming off. it looks like it has four slots and looks like some kind socket could you tell me what it is called because it is hard to explain.

Answer
It's called a sprag nut.  There is a special tool you need to undo it, or you can make a tool by using a suitable piece of pipe.  Buying the tool saves a lot of time, the part number is known by the dealer or is described in the manual.

Good luck,  Jan