Sudden surge

ONRUSH

Think of a rapid, overwhelming forward movement often used to describe water or crowds.

"Sudden surge" directly hints at a fast, forceful movement, which is exactly what "onrush" means—a rapid forward push or flood of something. The word itself combines "on" (indicating forward motion) with "rush" (a quick, intense burst), making it a straightforward definition. There’s no hidden wordplay here; the clue simply matches the answer’s literal meaning.