Article more about proposition (7)
I believe the answer is:
theorem
'proposition' is the definition.
(I know that theorem is a type of proposition)
'article more about' is the wordplay.
'article' becomes 'the' (the definite article).
'about' indicates an anagram.
'more' anagrammed gives '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" .)