Backless shoe (4)
I believe the answer is:
mule
'backless shoe' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mule that I've seen before include "Slipper - beast of burden" , "Crossbreed" , "Offspring of a donkey and a mare" , "Obstinate creature" , "Drug smuggler" .)