Intensify, with “up”

REV …

Think of what you might do to an engine or a party to make it more lively—it starts with "REV" and implies increasing energy or speed.

The clue plays on the phrase "rev up," which means to intensify or increase speed—like revving an engine. "Up" is given in the clue itself, so you just need the first part: "rev." The answer is shortened to "REV" to fit the crossword’s space, dropping the "up" since it’s already hinted at.