Criminal group (4)
'criminal group' is the definition.
(a criminal gang)
This is the entire clue.
(Other definitions for gang that I've seen before include "Group (crazy or criminal?)" , "Go to the Scots" , "Band of people, usually criminals" , "set of tools" , "Organised group of labourers" .)
(Other definitions for triad that I've seen before include "Chinese secret society" , "Set of three; chord" , "Three-note chord -- Chinese criminal gang" , "Union of three" , "three making a sound?" .)