Do you have any small picks? You can usually find them at most hardware stores. Harbor Freight has a set of 4 for a couple bucks usually if you don't want to invest too much into them. They are helpful for tricky parts like this and you can use them to dig at all sorts of things on your RC cars. Maybe wear safety glasses in case you pop it loose and it shoots off at your face. Also, rather than trying to bring it to the front of the drive cup so you can remove it, maybe push it back towards the diff. Then once you remove that drive cup you can get the C clip again.