Beastly sound? No sound! (5)
I believe the answer is:
neigh
'beastly sound?' is the definition.
(sound of a horse)
'no sound' is the wordplay.
'no' becomes 'nay' (I've seen this before).
'sound' shows a homophone (sound like).
'nay' sounds like 'NEIGH'.
(Other definitions for neigh that I've seen before include "Whinnying sound" , "Comment from Arab" , "Beastly sound" , "Whinnying noise" , "Hinge (anag.)" .)