Top up

FILL

Think of what you might do to a glass that's running low or a tank that's nearly empty.

Top up" is a phrase that means to add more of something until it's full, like refilling a glass or a gas tank. The answer "FILL" directly captures that idea—it’s the action of making something complete or full. There’s no hidden wordplay here; it’s a straightforward definition where the clue and answer align perfectly in meaning. Think of it as the simple, one-word command for "make it full.