Braying beast (5)
'braying beast' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ass that I've seen before include "animal inside" , "A donkey or an idiot" , "Braying animal" , "Charlie" , "Animal related to horses" .)
(Other definitions for burro that I've seen before include "Small donkey (US)" , "Spanish donkey" , "Beast of burden" , "Small donkey used as a pack animal" .)