We hear no sound from a horse (5)
I believe the answer is:
neigh
'sound from a horse' is the definition.
(I've seen this before)
'we hear no' is the wordplay.
'we hear' indicates a 'sounds like' (homophone) clue.
'no' becomes 'nay' (I've seen this before).
'nay' is a homophone of 'NEIGH'.
(Other definitions for neigh that I've seen before include "Beastly sound" , "High-pitched cry of a horse" , "Comment from stallion" , "Hinge (anag.)" , "Whinnying sound" .)