Plain old declaration of intent (9)

I believe the answer is:

manifesto

'declaration of intent' is the definition.

'plain old' is the wordplay.
'plain' becomes 'manifest' ('manifest' can be a synonym of 'plain'**).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'manifest'+'o'='MANIFESTO'

(Other definitions for manifesto that I've seen before include "electoral programme" , "statement of policy" , "Declaration of political programme" , "party programme" , "Public declaration of intent" .)

I've seen this clue in The Independent.

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