Champion Mark crowning America's old historian (9)
I believe the answer is:
herodotus
'historian' is the definition.
(Greek historian)
'champion mark crowning america's old' is the wordplay.
I cannot quite see how this works, but
'champion' could be 'hero' (synonyms) and 'hero' is found within the answer.
'mark' could be 'dot' (dotting is a kind of marking) and 'dot' is present in the answer.
'america's' could be 'us' ('US' can be a synonym of 'america') and 'us' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for herodotus that I've seen before include "Famous Greek" , "Old Greek" , "Historical pioneer" , "Ancient Greek historian" .)