Stubborn animals
I believe the answer is:
mules
'stubborn animals' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mules that I've seen before include "Offspring of male donkeys and female horses" , "Hybrid offsprings of a male donkey and a female horse" , "Stubborn horse-donkey animaly" , "Backless slippers" , "Beasts of burden" .)