Knock back the usual (3)
I believe the answer is:
rap
'knock' is the definition.
('rap' can be a synonym of 'knock')
'back the usual' is the wordplay.
'back' says the letters should be written in reverse.
'the usual' becomes 'par'.
'par' in reverse letter order is 'RAP'.
(Other definitions for rap that I've seen before include "Small amount" , "Form of music with spoken lyrics" , "talking during musical performance" , "Blame, punishment" , "Criminal charge" .)