Those people hiding potentially valuable mass proposition (7)
I believe the answer is:
theorem
'proposition' is the definition.
(theorem is a kind of proposition)
'those people hiding potentially valuable mass' is the wordplay.
'those people' becomes 'them' (I've seen this before).
'hiding' indicates putting letters inside.
'potentially valuable mass' becomes 'ore' (an ore might contain valuable minerals).
'them' placed around 'ore' is 'THEOREM'.
(Other definitions for theorem that I've seen before include "Deduced formula" , "Proof" , "deduction" , "Logical proposition" , "Mathematical statement to be proved" .)