Slip-on shoe (4)
(Other definitions for mule that I've seen before include "Obstinate quadruped" , "Sterile offspring of donkey and horse" , "Illegal drugs courier" , "Hybrid beast" , "Drug courier; animal" .)
'slip-on shoe' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for loafer that I've seen before include "maybe put foot in it" , "Casual footwear" , "One habitually putting feet up" , "Comfortable shoe or a do-nothing" , "One shirking work" .)
(Other definitions for moccasin that I've seen before include "Put one's foot in it" , "Slipper; snake" , "Venomous American snake" , "Soft leather 11" , "Soft, slip-on shoe - snake" .)