Square section circlip fits in a groove machined in the drive (half) shaft to stop inward movement of the joint. Used.
Seven advises you to consider upgraded drive shafts in a high-powered Mini. This is because the groove for this circlip is a weak point in the drive shaft, and under power, the shaft can snap at the bottom of this groove. See SSS0046 and SSS0047 to eliminate this inherent weakness.