The most common and most likely answer to this crossword clue is the 10 letter word FEDERATION
I've seen this crossword clue in The Guardian
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Buddy explains!
The straight part
We know that "Coalition" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case coalition = federation.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 FEDERATION mean?
FEDERATION noun- Act of joining together into a single political entity.
- Array of nations or states that are unified under one central authority which is elected by its members.
- Any society or organisation formed from separate groups or bodies.
- A collection of network or telecommunication providers that offer interoperability.
- Of an architectural style popular around the time of federation.