Article more about proposition (7)
I believe the answer is:
theorem
'proposition' is the definition.
(theorem is a kind of proposition)
'article more about' is the wordplay.
'article' becomes 'the' (the definite article in English).
'about' is an anagram indicator.
'more' is an anagram of 'orem'.
'the'+'orem'='THEOREM'
(Other definitions for theorem that I've seen before include "Proof" , "Proved mathematical hypothesis" , "Statement to be proved in maths" , "this has been established" , "The more it changed to a proposition to be proved" .)