End of series is nothing great (5)
I believe the answer is:
omega
'end of series' is the definition.
(last letter of the Greek alphabet)
'nothing great' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'great' becomes 'mega' (old-fashioned UK slang for 'very good').
'o'+'mega'='OMEGA'
'is' acts as a link.
(Other definitions for omega that I've seen before include "end of series" , "Greek finalist" , "character in Greek literature?" , "Traditional story" , "character from Athens" .)