Greek, Roman or Hebrew flag (7)
I believe the answer is:
ancient
'greek' is the definition.
(Ancient Greek is an example)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ancient that I've seen before include "Antique, old" , "Old salt in Coleridge's poem" , "Bygone" , "Very old, like the poetic mariner" , "Very old, belonging to times long ago" .)