Former scheme a politician is clarifying (11)
I believe the answer is:
explanatory
'is clarifying' is the definition.
(I've seen this before)
'former scheme a politician' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'scheme' becomes 'plan' (synonyms).
'politician' becomes 'tory' (term for a Conservative Party politician).
'ex'+'plan'+'a'+'tory'='EXPLANATORY'
(Other definitions for explanatory that I've seen before include "illustrative" , "Serving to make something clear" , "Clarifying" .)