Variety of bark?

ARF

Think of the sound a dog makes when it's excited, not the outer covering of a tree.

This clue plays on the double meaning of "bark." One definition is the outer covering of a tree, but it also refers to the sound a dog makes. The word "variety" hints at a different take on "bark," leading to the onomatopoeic "arf," which is a common way to spell a dog's bark. The question mark signals that it’s a pun, not a literal definition.