Sick American with one possible fantasy (8)
I believe the answer is:
illusion
'fantasy' is the definition.
(I know that fantasy can be written as illusion)
'sick american with one possible' is the wordplay.
'sick' becomes 'ill' (similar in meaning).
'american' becomes 'us' (abbreviation for United States).
'with' means one lot of letters go next to another.
'one' becomes 'I' (Roman numeral).
'possible' becomes 'on' ('not on' can mean unacceptable or not possible).
'ill'+'us'+'i'+'on'='ILLUSION'
(Other definitions for illusion that I've seen before include "A false belief about linus oil" , "Subjects under discussion" , "Deceptive impression" , "Fantasy or mirage" , "can you believe it?" .)