Be part of the crowd, with "in"

FIT

Think of how a piece might snugly belong in a larger whole, like a puzzle or a group where everything aligns just right.

The clue plays on the phrase "fit in," meaning to belong or blend with a group—literally being "part of the crowd." The word "in" is included right in the clue to hint at adding it to "fit," which gives you the answer. It’s a simple charade where "fit" + "in" would normally form "fit in," but here "in" is already accounted for, leaving just "fit" as the solution.