Hear “no,” a beastly sound (5)
I believe the answer is:
neigh
'a beastly sound' is the definition.
(sound of a horse)
'hear no' is the wordplay.
'hear' indicates a 'sounds like' (homophone) clue.
'no' becomes 'nay' (I've seen this before).
'nay' sounds like 'NEIGH'.
(Other definitions for neigh that I've seen before include "High-pitched cry of a horse" , "sound from the stable?" , "Comment from stallion" , "Animal noise (expressing 3?)" , "Mare's cry, e.g." .)