The most common and most likely answer to this crossword clue is the 11 letter word CONFEDERACY
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More clues leading to the result CONFEDERACY
Buddy explains!

The straight part
We know that "League" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case league = confederacy.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 CONFEDERACY mean?
CONFEDERACY noun- An alliance.
- A state where the sovereign constituent units delegate their authority to the centre. As opposed to a federation, where the central and regional governments are each equal and sovereign in their own sphere.
- Specifically, an instance of a decentralized governing structure among the indigenous peoples of North America.