*Forming a union

ORGAN

Think of a body part that also describes a structured group working together.

The clue "Forming a union" hints at the word "organ" because an organ is part of a living organism, which is essentially a union of cells working together. It also plays on the idea of an "organ" as a musical instrument, where multiple pipes combine to create harmony—another kind of union. The double meaning makes the answer fit neatly.