No sound from horse is heard (3)
I believe the answer is:
nay
'no' is the definition.
(I've seen this before)
'sound from horse is heard' is the wordplay.
'sound from horse' becomes 'neigh'.
'is heard' shows a homophone (sound like).
'neigh' is a homophone of 'NAY'.
(Other definitions for nay that I've seen before include "disagreement" , "Indeed" , "No - a negative vote" , "Negative response" , "Denial" .)