Knock art made with a couple of bits of junk (3-1-3-3)
I believe the answer is:
rat-a-tat-tat
'knock' is the definition.
'art made with a couple of bits of junk' is the wordplay.
'made with' indicates an anagram.
'couple of' shows that letters should be duplicated.
'bits of junk' becomes 'tat' (I am not sure about this - if you are sure you should believe this answer much more).
'tat' doubled is 'tattat'.
'art' with its letters mixed up with 'a' can make 'rata'.
'rata'+'tattat'='RAT-A-TAT-TAT'
(Another definition for rat-a-tat-tat that I've seen is " Knocking".)