B in test characteristic of a certain group (6)
I believe the answer is:
Here is my best explanation:
'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 traditional social group", "Clannish", "Like many primitive societies", "Belonging to social grouping", "Relating to kinship groups", "Of kinship groups", "Relating to a social group", "family" and "belonging to close-knit group".)