Letter -- nothing great (5)
I believe the answer is:
omega
'letter' is the definition.
(omega is a kind of letter)
'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 "16 letter" , "Last letter of the Greek alphabet" , "symbol of resistance?" , "Traditional story" , "Art of story-telling by body movement" .)