Code cracked by unspecified number of people in a group (6)
I believe the answer is:
ethnic
'of people in a group' is the definition.
(relating to an ethnic group)
'code cracked by unspecified number' is the wordplay.
'code' becomes 'ethic' (a moral code).
'cracked by' indicates putting letters inside (to crack can mean to create a divide in).
'unspecified number' becomes 'n' (n can mean any number).
'ethic' placed around 'n' is 'ETHNIC'.
(Other definitions for ethnic that I've seen before include "Relating to a particular culture" , "Characteristic of a certain culture" , "To do with racial group or culture" , "Relating to cultural divisions" , "Exotic" .)