Knocked over computers, making racket (4)
I believe the answer is:
scam
'racket' is the definition.
(racket is a type of scam or scheme)
'knocked over computers' is the wordplay.
'knocked over' says the letters should be written in reverse.
'computers' becomes 'Macs' (Apple Macs).
'macs' backwards is 'SCAM'.
'making' is the link.
(Other definitions for scam that I've seen before include "Swindle, fraud (colloq.)" , "Fraudulent action" , "A modern kind of swindle" , "Dubious operation" , "A slangy swindle or fraud" .)