Stallion's sound (5)
I believe the answer is:
This clue could be a double definition.
'stallion's' is the first definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'sound' is the second definition.
(neigh is a kind of sound)
Can you help me to learn more?
(Other definitions for neigh that I've seen before include "sound from the stable?", "heard on the farm", "Hinge (anag.)", "one made by Bucephalus?", "Whicker".)