The most common and most likely answer to this crossword clue is the 8 letter word PAIRINGS
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The straight part
We know that "Unions" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unions = pairings.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 PAIRINGS mean?
PAIRINGS noun- The combination or union of two things.
- An agreement between two members of a legislative body holding opposite opinions to refrain from voting, so that both may absent themselves.