The most common and most likely answer to this crossword clue is the 6 letter word SPLICE
I've seen this crossword clue in Financial Times
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More clues leading to the result SPLICE
- Joint left covered in cloves, say
- Marry childless learner driver, 18
- Join priest cutting piece of cake?
- Power cut’s welcomed? Form a union
- A bit of a bat, making 50 mid season?
- Join Labour’s leader to get into something exciting?
- Marry and relish holding party
- Marry, having a piece of cake with soft filling
- Tie problem for the head after schoolkids protest in principle
- Join up at last, cutting share
Buddy explains!

The straight part
We know that "Form a union" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case form a union = splice.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SPLICE mean?
SPLICE noun- A junction or joining of ropes made by splicing them together.
- The electrical and mechanical connection between two pieces of wire or cable.
- That part of a bat where the handle joins the blade.
- Bonding or joining of overlapping materials.
- The process of removing intron sequences from the pre-messenger RNA, and then joining together exons.
- To unite, as two ropes, or parts of a rope, by a particular manner of interweaving the strands, -- the union being between two ends, or between an end and the body of a rope.
- To unite, as spars, timbers, rails, etc., by lapping the two ends together, or by applying a piece which laps upon the two ends, and then binding, or in any way making fast.
- To unite in marriage.
- To unite as if splicing.
- To remove intron sequences from the pre-messenger RNA, and then join together exons.