Prediction revealed by American chap in dream (9)
I believe the answer is:
horoscope
'prediction' is the definition.
(horoscope is a kind of prediction)
'american chap in dream' is the wordplay.
'american chap' becomes 'rosco' (American name).
'in' indicates putting letters inside.
'dream' becomes 'hope'.
'rosco' placed within 'hope' is 'HOROSCOPE'.
'revealed by' is the link.
(Other definitions for horoscope that I've seen before include "special map" , "Forecast of fortune" , "An astrological forecast" , "Forecast of a person's future based on the heavens" , "Prediction based on one's star sign" .)