Greek letter is nothing fantastic (5)
I believe the answer is:
omega
'greek letter' is the definition.
(letter from the Greek alphabet)
'nothing fantastic' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'fantastic' becomes 'mega' (I've seen this before).
'o'+'mega'='OMEGA'
'is' acts as a link.
(Other definitions for omega that I've seen before include "letter to Greeks" , "Last (Gk. letter)" , "Consummation" , "16 letter" , "Conclusion" .)