Last part of a series is nothing brilliant (5)
I believe the answer is:
omega
'last part of a series' is the definition.
(last Greek letter so last of a series by extension)
'nothing brilliant' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'brilliant' becomes 'mega' (old-fashioned UK slang for 'very good').
'o'+'mega'='OMEGA'
'is' is the link.
(Other definitions for omega that I've seen before include "symbol of resistance?" , "end of series" , "character in Greek literature?" , "in Greece, the limit!" , "24th Greek letter" .)