Whinnies (6)
I believe the answer is:
neighs
'whinnies' is the definition.
('neigh' can be a synonym of 'whinny')
This is all the clue.
(Other definitions for neighs that I've seen before include "Makes noise like horse" , "Makes whinnying sound like horse" , "beastly sounds" , "Sounds made by a horse" , "Makes noises like a horse" .)