Principle of 1 with old money at the start (5)
I believe the answer is:
axiom
'principle' is the definition.
(an axiom is a belief or principle)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for axiom that I've seen before include "Established principle" , "Plainly true proposition" , "fundamental principle" , "Old saying or precept 95)" , "Self-evident principle" .)