Last letter's nothing great (5)
I believe the answer is:
omega
'last letter's' is the definition.
(I've seen this before)
'nothing great' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'great' becomes 'mega' (I've seen this before).
'o'+'mega'='OMEGA'
(Other definitions for omega that I've seen before include "letter to Greeks" , "Ending of a series or sequence" , "Athenian character" , "Traditional story" , "character in Greek literature?" .)