Footwear — people who smuggle drugs (5)

I believe the answer is:

mules

(Other definitions for mules that I've seen before include "Stubborn horse-donkey animaly" , "Offspring of male donkeys and female horses" , "Beasts of burden" , "Hybrid offsprings of a male donkey and a female horse" , "Backless slippers" .)

I've seen this clue in The Guardian.

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