Great ruler learned tax must change, cutting tons (9)

I believe the answer is:

alexander

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'great ruler' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for alexander that I've seen before include "A great Greek ruler?" , "Macedon's "great" king, d. 323 BC" , "Fellow" , "band-leader" , "Band leader" .)

I've seen this clue in The Telegraph.

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