Horse’s whinny (5)
I believe the answer is:
neigh
'horse's whinny' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for neigh that I've seen before include "Cry coming from mount" , "Whinnying sound" , "heard on the farm" , "Animal noise (expressing 3?)" , "The noise of a bay" .)