To knock a popular song (3)
I believe the answer is:
hit
This clue is a double definition.
'to knock' is the first definition.
('hit' can be a synonym of 'knock')
'a popular song' is the second definition.
(a hit song)
(Other definitions for hit that I've seen before include "Successful record/film" , "Strike, clout" , "Successful song" , "Murder" , "Rap" .)