Sharp knock at the door of opium dens outside studio apartment
I believe the answer is:
rat-tat
'sharp knock at the door' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for rat-tat that I've seen before include "Rapping sound of door knocker" , "Sharp noise of a knock on the door" , "Short sharp noises of a knock on the door" , "Sound of a knock on the door" , "Knocking sound" .)