Equine refusal? (5)

I believe the answer is:
neigh
'equine refusal?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
This is the entire clue.
Can you help me to learn more?
(Other definitions for neigh that I've seen before include "Comment from stallion" , "Farmyard sound" , "one made by Bucephalus?" , "Horse's noise" , "Mare's cry, e.g." .)
