Donkey's bray (6)
I believe the answer is:
heehaw
'donkey's bray' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for heehaw that I've seen before include "Imitation of a donkey's sound" , "Horse-laugh" , "Ass's bray" , "Cry of a mule" , "Written bray of a donkey" .)