One doesn't believe in roast chicken? (7)
I believe the answer is:
heathen
'one doesn't believe' is the definition.
(term for a non-religious person)
'roast chicken?' is the wordplay.
'roast' becomes 'heat' (roasting is a method of heating or cooking).
'chicken?' becomes 'hen' (hen is a kind of chicken).
'heat'+'hen'='HEATHEN'
'in' is the link.
(Other definitions for heathen that I've seen before include "Person lacking culture or moral principles" , "Unbeliever" , "Uncultured" , "Savage" , "One who doesn't acknowledge your god" .)