Unbeliever is at the assembly (7)
I believe the answer is:
atheist
'unbeliever' is the definition.
(atheist is a kind of unbeliever)
'is at the assembly' is the wordplay.
'assembly' indicates an anagram.
'is'+'at'+'the'='isatthe'
'isatthe' with letters rearranged gives 'ATHEIST'.
(Other definitions for atheist that I've seen before include "One who disbelieves the existence of gods" , "One who does not believe in the existence of a god" , "Godless person" , "I don't believe" , "One denies the existence of a god" .)