The most common and most likely answer to this crossword clue is the 7 letter word TUPPING
T
U
P
P
I
N
G
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The straight part
We know that "Act of Union" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case act of union = tupping.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 TUPPING mean?
TUPPING verb- To mate; used of a ram mating with a ewe.
- To have sex with, to bonk, etc.
- (regional English) To butt: said of a ram.