He doesn't believe two articles first (7)
I believe the answer is:
atheist
'he doesn't believe' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'two articles first' is the wordplay.
'two articles' means two replacements for 'articles'.
'articles' says to put letters next to each other.
'articles' becomes 'a the' ('a' and 'the', indefinite and definite articles).
'first' becomes 'ist' (ist resembles 1st).
'athe' put next to 'ist' is 'ATHEIST'.
Can you help me to learn more?
(Other definitions for atheist that I've seen before include "God denier" , "One who disbelieves the existence of gods" , "One denying existence of a god" , "Disbeliever in gods" , "I don't believe" .)