Assumed American chose to support Democrat (7)
I believe the answer is:
adopted
'assumed' is the definition.
('adopt' can be a synonym of 'assume')
'american chose to support democrat' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'chose' becomes 'opted' (to opt for something is to choose it).
'to support' says to put letters next to each other (in a down clue, some letters hold up others).
'democrat' becomes 'd'.
'opted' after 'd' is 'dopted'.
'a'+'dopted'='ADOPTED'
(Other definitions for adopted that I've seen before include "raised issue on behalf of others" , "Took into one's family" , "Chose to follow" , "Took under one's wing" , "Taken as one's own" .)