The Witcher 2: Assassins of Kings[]
While Nekker claws are mainly used in alchemy, they can be used as one of the components needed to craft the Armor of Ys and Blasphemer's armor.
The Witcher 3: Wild Hunt[]
Nekker claw is an alchemy ingredient. It is not needed to craft anything but can be dismantled into monster claw.
It is a random drop from nekkers, their monster nests, and nekker warriors.