Athenian letter, old and great (5)
I believe the answer is:
omega
'athenian letter' is the definition.
(Greek letter)
'old and great' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'and' says to put letters next to each other.
'great' becomes 'mega' (I've seen this in other clues).
'o'+'mega'='OMEGA'
(Other definitions for omega that I've seen before include "24th Greek letter" , "Athenian character" , "long letter" , "Ultimate letter" , "Conclusion" .)