Clear old platform (9)
I believe the answer is:
manifesto
'platform' is the definition.
(a political platform)
'clear old' is the wordplay.
'clear' becomes 'manifest' (synonyms).
'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" , "Political statement of intent" , "electoral programme" , "Statement of beliefs" , "Party's publicly declared aims" .)