Supreme as Ali claimed to be (8)

I believe the answer is:

greatest

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'supreme' is the definition.
(thesaurus)

I don't understand the remainder of the clue.

(Other definitions for greatest that I've seen before include "Get stare from the mightiest" , "Unsurpassed" , "Biggest and best, like Muhammad Ali" , "Highest in quality or in quantity" , "Unsurpassable" .)

I've seen this clue in The Sun.

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