Extreme to attempt a filibuster, perhaps (9)

I believe the answer is:

uttermost

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'extreme' is the definition.
(I know that extreme can be written as uttermost)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for uttermost that I've seen before include "at the furthest limit" , "The furthest point or degree" , "Greatest possible extent" , "Extreme" , "best" .)

I've seen this clue in The Independent.

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