Superstar (4)
(Other definitions for alien that I've seen before include "Foreign - film directed by Ridley Scott" , "of foreigners" , "repugnant" , "Martian, say" , "coming from overseas" .)
'superstar' is the definition.
(idol can mean a celebrity)
This is all the clue.
(Other definitions for idol that I've seen before include "pop star" , "Who's greatly respected" , "Object of worship (or love?)" , "A graven image, or a hero" , "Worshipped figure (not painting); revered person" .)