Non-believer (7)
'non-believer' is the definition.
(someone who doesn't believe in God)
This is all the clue.
(Other definitions for atheist that I've seen before include "One believes there are no gods" , "One not believing in gods" , "One disbelieving in the existence of God or gods" , "Disbeliever in gods" , "One who believes there is no god" .)
(Other definitions for infidel that I've seen before include "Someone who does not acknowledge your god" , "One who doesn't believe" , "Literally, 9 [UNFAITHFUL]" , "Non-believer, heretic" , "Person who does not believe in a religion" .)