Obvious old declaration (9)
I believe the answer is:
manifesto
'declaration' is the definition.
(I know that declaration can be written as manifesto)
'obvious old' is the wordplay.
'obvious' becomes 'manifest' (similar in meaning).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'manifest'+'o'='MANIFESTO'
(Other definitions for manifesto that I've seen before include "Pre-election promises" , "statement of policy" , "Declaration of political programme" , "Political statement of intent" , "electoral programme" .)