Edge of pound coin assigned this, originally Latin as agreed? (5,2,7)

I believe the answer is:
decus et tutamen
'originally latin as agreed?' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'edge of pound coin assigned' is the wordplay.
I cannot quite see how this works, but
'coin' could be 'd' (old British penny abbreviation) and 'd' is present in the answer.
'assigned' could be 'set' (I've seen this in another clue) and 'set' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'this' acts as a link.
Can you help me to learn more?
(Another definition for decus et tutamen that I've seen is " Words on some pound coins".)
