The most common and most likely answer to this crossword clue is the 6 plus 8 letter word UNHOLY ALLIANCE
I've seen this crossword clue in The Daily Telegraph
U
N
H
O
L
Y
A
L
L
I
A
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C
E
More clues leading to the result UNHOLY ALLIANCE
Buddy explains!
The straight part
We know that "Rivals in union" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rivals in union = unholy alliance.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.