Arsenic and chicken pasty (5)
I believe the answer is:
ashen
'pasty' is the definition.
(both can mean pale)
'arsenic and chicken' is the wordplay.
'arsenic' becomes 'As' (As is the chemical symbol for arsenic).
'and' is a charade indicator (letters next to each other).
'chicken' becomes 'hen' (hen is a kind of chicken).
'as'+'hen'='ASHEN'
(Other definitions for ashen that I've seen before include "White-faced" , "Like bird" , "Unhealthily pale" , "Livid" , "pasty" .)