The bit in the centre is the circlip (arrow in the pic below). Be careful when removing it as the whole assembly is under tension and will fly apart if you don't hold it together.
When you're putting it back together the easiest way is to remove the B-tension screw (the one that sits against the tab on the frame) and use that part to grip with a pair of pliers as you turn it to tension the spring.
When you're putting it back together the easiest way is to remove the B-tension screw (the one that sits against the tab on the frame) and use that part to grip with a pair of pliers as you turn it to tension the spring.