A fresh crab claw is an uncommon drop from crabs, which may be crafted into a crab claw with a chisel and 15 Crafting. Successfully doing so will grant 32.5 Crafting experience. However, there is a chance of failure which will result in a broken crab claw instead. The chance of successfully crafting a crab claw scales from 15% at level 1 Crafting to 94% at level 99 Crafting.
Recipe for Disaster must be partially completed for players to be able to access the Mogre Camp, where the crabs are located, and kill them.
For an exhaustive list of all known sources for this item, see here