Greek character old and huge (5)
I believe the answer is:
omega
'greek character' is the definition.
(letter from the Greek alphabet)
'old and huge' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'and' is a charade indicator (letters next to each other).
'huge' becomes 'mega' (I've seen this before).
'o'+'mega'='OMEGA'
(Other definitions for omega that I've seen before include "24th Greek letter" , "Conclusion" , "Foreign character" , "completes a series" , "Art of story-telling by body movement" .)