Unbelievers (8)
I believe the answer is:
atheists
'unbelievers' is the definition.
(atheist is a kind of unbeliever)
This is all the clue.
(Other definitions for atheists that I've seen before include "Heathens" , "Godless people" , "ungodly crew" , "Non-believers" , "They deny the existence of a god" .)