Greek character old and enormous (5)
I believe the answer is:
omega
'greek character' is the definition.
(letter from the Greek alphabet)
'old and enormous' is the wordplay.
'old' becomes 'o' (abbreviation).
'and' says to put letters next to each other.
'enormous' becomes 'mega ' (I've seen this before).
'o'+'mega'='OMEGA'
(Other definitions for omega that I've seen before include "in Greece, the limit!" , "Ultimate letter" , "Final Greek letter" , "16 letter" , "end of series" .)