This writer brought up to follow the Golden Rule? (7)
I believe the answer is:
theorem
'rule?' is the definition.
(I have seen 'Mathematical rule ' mean 'theorem' so perhaps 'rule' could also mean 'theorem')
'this writer brought up to follow the golden' is the wordplay.
'this writer' becomes 'me' (the person who wrote the clues).
'brought up' is a reversal indicator (in a down clue, letters go up).
'to follow' says to put letters next to each other (some letters go after or follow others).
'golden' becomes 'or' ('or' is the name for gold in heraldry).
'me' backwards is 'em'.
'the'+'or'='theor'
'em' after 'theor' is 'THEOREM'.
(Other definitions for theorem that I've seen before include "Proven hypothesis" , "The more it changed to a proposition to be proved" , "Demonstrable proposition" , "Mathematical formula" , "Geometric proposition" .)