This is a cryptic definition - the entire clue describes the answer in a clever, indirect way that requires you to think about the situation described.
Think about what happens to different parts of your body when you change from sitting to standing. What body part literally disappears or ceases to exist in this transition?
Answer: LAP | This is a riddle-style cryptic definition: your lap is a part of your body that only exists when you're sitting - it disappears when you stand up!