Slipper he imports on the sly (4)
I believe the answer is:
mule
'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" .)