Turning corrupt, a politician (8)
I believe the answer is:
rotatory
'turning' is the definition.
(I've seen this before)
'corrupt a politician' is the wordplay.
'corrupt' becomes 'rot'.
'politician' becomes 'tory' (I've seen this before).
'rot'+'a'+'tory'='ROTATORY'
(Other definitions for rotatory that I've seen before include "circular route?" , "Revolving about an axis" , "Going round" , "Turning like a wheel" , "Revolutionary" .)