"Dragon claw does work! It’s supposed to be incredible, really gives your brain a boost, you come over all cunning for a few hours –"
Ron Weasley[src]

Dragon claw was the claw of a dragon, and, when powdered down and eaten, gives the consumer a 'brain boost', which leaves the consumer cunning 'for hours'.

During the Hogwarts O.W.L.s and N.E.W.T.s examinations in 1996, powdered dragon claw was being sold amongst the fifth and seventh years in order to improve their exam results. It was offered to Hermione Granger by Harold Dingle, however, she confiscated it. This was later discovered to be dried Doxy droppings.