Seized power held by American, rude bastard (7)
I believe the answer is:
usurped
'seized' is the definition.
'power held by american rude bastard' is the wordplay.
'power' becomes 'p'.
'held by' is an insertion indicator.
'american' becomes 'us'.
'bastard' is an anagram indicator (bastard can mean impure or mixed).
'rude' is an anagram of '[rude]'.
'us'+'[rude]'='us[rude]'
'p' put within 'usured' is 'USURPED'.
(Other definitions for usurped that I've seen before include "Seized power illegally" , "Taken by force" , "Took power illegally" , "Took power or position wrongfully" , "Seized and took control" .)