Negative to sound like a horse (3)
I believe the answer is:
nay
'negative' is the definition.
(nay is a kind of negative)
'sound like a horse' is the wordplay.
I cannot really see how this works, but
'like' could be 'y' (similar in meaning) and 'y' is found in the answer.
'a' is found in the answer.
A single letter 'n' remains which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for nay that I've seen before include "Refusal" , "Archaic negative" , "Denial" , "Negative answer" , "No - a negative vote" .)