Article more about idea (7)
I believe the answer is:
theorem
'idea' is the definition.
(theorem is a kind of idea)
'article more about' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech).
'about' indicates anagramming the letters.
'more' with letters rearranged gives 'orem'.
'the'+'orem'='THEOREM'
(Other definitions for theorem that I've seen before include "(Maths) proposition" , "A rule in algebra" , "Geometrical proposition to be proved" , "Algebraic rule" , "Mathematical proposition proved by reasoning" .)