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