Key Greek character in old Greek city (6)
I believe the answer is:
delphi
'old greek city' is the definition.
(I've seen this before)
'key greek character' is the wordplay.
'key' becomes 'del' (Delete on a keyboard).
'greek character' becomes 'phi' (letter from the Greek alphabet).
'del'+'phi'='DELPHI'
'in' is the link.
(Other definitions for delphi that I've seen before include "Site of classical Greek oracle" , "Where Apollo's most famous oracle spoke in Greece" , "Place of the famous oracle" , "Ancient Greek town with 21" , "Temple site on Mt Parnassus" .)