No horse's sound can be heard (3)
I believe the answer is:
nay
'no' is the definition.
(I've seen this before)
'horse's sound can be heard' is the wordplay.
'horse's sound' becomes 'neigh' (I've seen this before).
'can be heard' shows a homophone (sound like).
'neigh' sounds like 'NAY'.
(Other definitions for nay that I've seen before include "even" , "Refusal" , "Negative response" , "In point of fact - a parliamentary vote against" , "No (dialect); or rather" .)