Top actor knocking scoundrels over (4)
I believe the answer is:
star
'top actor knocking' is the definition.
'star' can be an answer for 'top' (eg a top player is a star player). I also know that 'actor' relates to this answer.
'scoundrels over' is the wordplay.
'scoundrels' becomes 'rats' (I've seen this in another clue).
'over' says the letters should be written in reverse.
'rats' written backwards gives 'STAR'.
(Other definitions for star that I've seen before include "Brilliant performer?" , "For example, Marilyn Monroe" , "Five-pointed celestial body" , "Famous entertainer" , "top player" .)