Old colossal Greek character (5)
I believe the answer is:
omega
'greek character' is the definition.
(letter from the Greek alphabet)
'old colossal' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'colossal' becomes 'mega' (I can't explain this - if you can you should believe this answer much more).
'o'+'mega'='OMEGA'
Can you help me to learn more?
(Other definitions for omega that I've seen before include "character in Greek literature?" , ""The Alpha and the ___"" , "Consummation" , "Ultimate letter" , "Ending of a series or sequence" .)