The gold measure - a demonstrable principle (7)
I believe the answer is:
theorem
'a demonstrable principle' is the definition.
'theorem' can be an answer for 'principle' (I've seen this before). I'm not sure about the 'a demonstrable' bit.
'the gold measure' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'measure' becomes 'em' (em is a kind of measure).
'the'+'or'+'em'='THEOREM'
Can you help me to learn more?
(Other definitions for theorem that I've seen before include "Proof" , "Geometrical proposition to be proved" , "Demonstrable proposition" , "Geometric proposition" , "Proven hypothesis" .)