Protest — Conservative rotter is a US politician (8)
I believe the answer is:
'a us politician' is the definition.
(I've seen this before)
'protest conservative rotter' is the wordplay.
'protest' becomes 'demo' (I've seen this before).
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'rotter' becomes 'rat' ('rat' can be a synonym of 'rotter').
'is' acts as a link.
(Other definitions for democrat that I've seen before include "US politician" , "Advocate of government by elected representatives" , "One who believes in government by the whole people" , "American politician" , "Political type" .)