Emperor once gambling with cards, said flier? (9)

I believe the answer is:

montezuma

I'm a little stuck... Click here to teach me more about this clue!

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

I don't understand the remainder of the clue.

(Other definitions for montezuma that I've seen before include "stomach-turning revenge" , "Old American leader" , "Aztec emperor (who takes his revenge on tourists?)" , "Aztec "revenge" ruler" , "Last Aztec emperor (who now has his revenge!)" .)

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

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