Greek character is nothing fantastic (5)
I believe the answer is:
omega
'greek character' is the definition.
(letter from the Greek alphabet)
'nothing fantastic' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'fantastic' becomes 'mega' (I've seen this before).
'o'+'mega'='OMEGA'
'is' is the link.
(Other definitions for omega that I've seen before include "character from Athens" , "letter to Greeks" , "16 letter" , "Foreign character" , "Last (Gk. letter)" .)