Obvious nothing is a declaration (9)
I believe the answer is:
manifesto
'declaration' is the definition.
(I know that declaration can be written as manifesto)
'obvious nothing' is the wordplay.
'obvious' becomes 'manifest' (similar in meaning).
'nothing' becomes 'o' (looks like zero - 0).
'manifest'+'o'='MANIFESTO'
'is a' acts as a link.
(Other definitions for manifesto that I've seen before include "Public policy declaration" , "Public declaration of intent" , "Publication containing political programme" , "Declaration of political programme" , "Statement of beliefs" .)