Sickening sound of a chicken (4)
I believe the answer is:
foul
'sickening' is the definition.
(synonyms)
'sound of a chicken' is the wordplay.
'sound of' shows a homophone (sound like).
'a chicken' becomes 'fowl' (I've seen this before).
'fowl' sounds like 'FOUL'.
(Other definitions for foul that I've seen before include "Offensive; make dirty" , "Vile" , "Putrid" , "Abhorrent" , "Repulsive" .)