Start of a popular saying about gravity ... or a hint to completing the answers to the starred clues

WHATGOESUP

Think of Newton’s discovery and the first half of a well-known phrase about things returning.

This clue plays on the famous phrase "what goes up must come down," which refers to gravity. The answer is just the first part of that saying—"WHATGOESUP"—while also hinting that the starred clues in the puzzle need "MUSTCOMEDOWN" added to their answers to make sense. It’s a clever way to tie the theme together by splitting a well-known expression across multiple clues.