In the NYT Crossword, “Forming a union” commonly clues ORGANIZING, referring to workers organizing a labor union. Depending on context and letter count, it can also be UNITING or MARRYING, but organizing is the most frequent crossword answer.
In crosswords by the NYT, forming a union generally clues ORGANIZING, i.e., the act of starting a labor union. Of course, there can be other answers depending on how many letters are needed, but this is by far the most frequent one.