Proposition of millions after article on mineral deposit (7)
I believe the answer is:
theorem
'proposition' is the definition.
(theorem is a kind of proposition)
'millions after article on mineral deposit' is the wordplay.
'millions' becomes 'm' (abbreviation).
'after' is a charade indicator (letters next to each other).
'article' becomes 'the' (the 'definite article' part of speech**).
'on' is a charade indicator (letters next to each other).
'mineral deposit' becomes 'ore' (I've seen this before).
'the'+'ore'='theore'
'm' put after 'theore' is 'THEOREM'.
'of' is the link.
(Other definitions for theorem that I've seen before include "Demonstrable proposition" , "Formula to be proved" , "Mathematical rule" , "Proven proposition" , "The more it changed to a proposition to be proved" .)