Dirty chicken, say (4)
I believe the answer is:
foul
'dirty' is the definition.
(I know that dirty can be written as foul)
'chicken say' is the wordplay.
'chicken' becomes 'fowl' (chicken is a type of fowl).
'say' indicates a 'sounds like' (homophone) clue.
'fowl' is a homophone of 'FOUL'.
(Other definitions for foul that I've seen before include "Offensive, abominable" , "Filthy" , "Vile" , "Abominable - offence on football field" , "Having offensive smell or taste" .)