Ring huge Greek character (5)
I believe the answer is:
omega
'greek character' is the definition.
(letter from the Greek alphabet)
'ring huge' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'huge' becomes 'mega' (mega can mean very large).
'o'+'mega'='OMEGA'
(Other definitions for omega that I've seen before include "completes a series" , "Consummation" , "Art of story-telling by body movement" , "Last thing" , "Traditional story" .)