Negative response heard from horse (5)
I believe the answer is:
neigh
'from horse' is the definition.
(noise from a horse)
'negative response heard' is the wordplay.
'negative response' becomes 'nay' (I've seen this before).
'heard' indicates a 'sounds like' (homophone) clue.
'nay' sounds like 'NEIGH'.
(Other definitions for neigh that I've seen before include "Comment from Arab" , "Horse's whinny" , "Horse's cry" , "nicker" , "Horse's noise" .)