American prude, cast assumed (7)
I believe the answer is:
usurped
'assumed' is the definition.
('usurp' can be a synonym of 'assume')
'american prude cast' is the wordplay.
'american' becomes 'US' (abbreviation for United States).
'cast' is an anagram indicator (letters are dispersed or cast around).
'prude' anagrammed gives 'urped'.
'us'+'urped'='USURPED'
(Other definitions for usurped that I've seen before include "Seized power illegally" , "Seized and took control" , "Taken by force" , "Took power by force" , "Took control (by force)" .)