B in test characteristic of a certain group (6)
I believe the answer is:
tribal
'characteristic of a certain group' is the definition.
'b in test' is the wordplay.
'in' is an insertion indicator.
'test' becomes 'trial' ('trial' can be a synonym of 'test').
'b' put inside 'trial' is 'TRIBAL'.
(Other definitions for tribal that I've seen before include "between extended families" , "Of a group of people related by blood" , "Clannish" , "Like certain social groups" , "Like some groups of people" .)