Humour mostly upset scoundrel, a US politician (8)
I believe the answer is:
democrat
'a us politician' is the definition.
(I've seen this before)
'humour mostly upset scoundrel' is the wordplay.
I cannot really understand how this works, but
'scoundrel' could be 'rat' (both can mean a dishonourable person) and 'rat' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for democrat that I've seen before include "American politician" , "US politician" , "One advocates government by the people's elected representatives" , "Believer in government of, for and by the people" , "Republican's US rival" .)