Not the forename of forty thieves! (3)

I believe the answer is:

ali

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'of forty thieves' is the definition.
The definition suggests a singular noun which matches the answer.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for ali that I've seen before include ""The Greatest" boxer" , "Mohammad . . ., . . . Hewson" , "Mohammed . . . (boxer)" , "Muslim name" , "Lepidopterous floater, apian stinger, d. 2016" .)

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