Offspring of a donkey and a horse (4)
I believe the answer is:
mule
'offspring of a donkey and a horse' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for mule that I've seen before include "Obstinate person?" , "Drug courier; animal" , "Hybrid animal - slipper" , "Donkey and horse cross" , "Obstinate creature" .)