Consider snakes, part of the vertebrates and, within that group, of the tetrapods. Even though they are part of the tetrapod clade, they have lost the four legs which help define that group. As another example, there are the caecilians, tetrapod amphibians which have also lost their legs during evolution.
Work Step by Step
Scan through the chapter and look for examples of groups in which some members lose a defining trait present in other members of the group. There are other correct answers for this question.