A debatable assembly? (4)

I believe the answer is:

moot

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'a debatable' is the definition.
(I know that debatable can be written as moot)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for moot that I've seen before include "Subject to dispute" , "Controversial" , "Such a point is arguable" , "Anglo-Saxon assembly; subject to debate" , "Such a point is disputable" .)

I've seen this clue in the Sydney Morning Herald.

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