Illegal enterprise making a lot of noise (6)
I believe the answer is:
racket
I believe this clue is a double definition.
'illegal enterprise' is the first definition.
(I've seen this in another clue)
'a lot of noise' is the second definition.
(I've seen this before)
'making' is the link.
(Other definitions for racket that I've seen before include "See 1" , "noise pollution" , "Infernal row" , "Uproar" , "Din; snowshoe" .)