Demarcated group rejecting leader in criminal enterprise (6)
I believe the answer is:
racket
'criminal enterprise' is the definition.
(I've seen this before)
'demarcated group rejecting leader' is the wordplay.
'demarcated group' becomes 'bracket' (eg those in a pay bracket).
'rejecting leader' means to remove the first letter.
'bracket' with its initial letter taken away is 'RACKET'.
'in' acts as a link.
(Other definitions for racket that I've seen before include "Lot of noise or bat for game (6)" , "Loud discordant noise" , "Din - strung bat" , "See 1" , "Din of the fraudulent moneymaking activity" .)