Group of three; Chinese criminal society (5)
I believe the answer is:
triad
This clue is a double definition.
'group of three' is the first definition.
(group of three things)
'chinese criminal society' is the second definition.
(I've seen this before)
(Other definitions for triad that I've seen before include "Could be major" , "Set of three; chord" , "Up to date" , "Chinese criminal society" , "Three-note chord -- Chinese criminal gang" .)