Whinnied (7)
I believe the answer is:
neighed
'whinnied' is the definition.
('neigh' can be a synonym of 'whinny')
This is all the clue.
(Other definitions for neighed that I've seen before include "Heeding (anag)" , "how Arab, for example, sounded" , "Cried" , "Whinnied" , "sounded "horsey"" .)