Sow's sound (4)
I believe the answer is:
oink
'sow's sound' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for oink that I've seen before include "Beastly noise" , "Piggy noise" , "Noise made by a pig" , "it might be in a pen one hears" , "what we hear from Berkshire?" .)