Neigh (6)
I believe the answer is:
whinny
'neigh' is the definition.
(I know that neigh can be written as whinny)
This is all the clue.
(Other definitions for whinny that I've seen before include "animal noise?" , "Horse's cry" , "A horse may do it" , "High-pitched neigh" , "Gentle or joyful neigh" .)