An aristocrat's old coin (5)
I believe the answer is:
noble
I believe this clue is a double definition.
'an aristocrat's' is the first definition.
(noble is a kind of aristocrat)
'old coin' is the second definition.
(I've seen this before)
(Other definitions for noble that I've seen before include "Of blue blood" , "Honourable ? aristocratic" , "Imposing, magnificent" , "with a title?" , "Illustrious - of high birth - old gold coin" .)