Actor with sheet sings from the bottom (4,4)
I believe the answer is:
film star
'actor' is the definition.
(film star is a kind of actor)
'with sheet sings from the bottom' is the wordplay.
'with' is a charade indicator (letters next to each other).
'sheet' becomes 'film' (film is a kind of sheet).
'sings' becomes 'rats' (both can mean 'acts as an informant').
'from the bottom' says the letters should be written backwards.
'rats' written backwards gives 'star'.
'film'+'star' is 'FILM-STAR'.
(Other definitions for film star that I've seen before include "Hollywood hero?" , "Ma flirts with Hollywood idol" , "Cinema celebrity" , "Cruise, say," , "Famous movie actor" .)