Knocking sound made by rodent alongside rubbish (3-3)
I believe the answer is:
rat-tat
'knocking sound' is the definition.
(I've seen this before)
'rodent alongside rubbish' is the wordplay.
'rodent' becomes 'rat' (rat is a kind of rodent).
'alongside' means one lot of letters go next to another.
'rubbish' becomes 'tat' (I've seen this before).
'rat'+'tat'='RAT-TAT'
'made by' acts as a link.
(Other definitions for rat-tat that I've seen before include "Rapping sound of door knocker" , "Knocking sound" , "Sound of a knock on the door" , "Sharp noise of a knock on the door" , "Sharp sound of a door-knocker" .)