The most common and most likely answer to this crossword clue is the 12 letter word COHABITATION
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Buddy explains!
The straight part
We know that "Arrangement of some partners/a style of government" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case arrangement of some partners/a style of government = cohabitation.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 COHABITATION mean?
COHABITATION noun- An emotional and physical intimate relationship which includes a common living place and which exists without legal or religious sanction.
- The act of living together.
- A place where two or more individuals reside together.
- The act of two species living together in the same habitat.
- Cooperation between politicians of opposing political parties; especially, in France, between a President and Prime Minister.