Nothing fantastic in Greek letter (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' (looks like zero - 0).
'fantastic' becomes 'mega' (I've seen this before).
'o'+'mega'='OMEGA'
'in' acts as a link.
(Other definitions for omega that I've seen before include "Art of story-telling by body movement" , "Last thing" , "The last in a series of letters" , "Athenian character" , "Traditional story" .)