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