Knock a self-service arrangement (6)
I believe the answer is:
buffet
'knock' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for buffet that I've seen before include "Wallop" , "Self-service meal; strike" , "place selling snacks" , "Refreshment counter - slap" , "Variety of restaurant" .)