Crossword Genius

Slipper he imports on the sly (4)

Ross

I believe the answer is:

mule

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'slipper' is the definition.
(mule is a kind of slipper)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for mule that I've seen before include "Illegal drugs courier" , "Crossbreed" , "Donkey and horse cross" , "Obstinate person?" , "Hybrid beast" .)

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