Evil American one on for fantasy (8)
I believe the answer is:
illusion
'for fantasy' is the definition.
(synonyms)
'evil american one on' is the wordplay.
'evil' becomes 'ill' (as in doing people ill).
'american' becomes 'US' (abbreviation for United States).
'one' becomes 'i' (Roman numeral).
'ill'+'us'+'i'+'on'='ILLUSION'
(Other definitions for illusion that I've seen before include "Subjects under discussion" , "Deceptive appearance" , "A false belief about linus oil" , "Delusion, fantasy" , "False vision or mirage (8)" .)