Mathematical rule (7)
I believe the answer is:
theorem
'mathematical rule' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for theorem that I've seen before include "Logical proposition" , "Statement to be proved in maths" , "Geometrical proposition to be proved" , "Mathematical statement to be proved" , "Hero met (anag) - principle that's proven" .)