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 subsidiary indication.
'in' is an insertion indicator.
'test' becomes 'trial' ('trial' can be a synonym of 'test'**).
'b' inserted within 'trial' is 'tribal'.
(Other definitions for tribal that I've seen before include "Belonging to social grouping", "Like many primitive societies", "Of racial group or clan", "Of a group of people related by blood", "Relating to a social group", "Relating to people related by blood", "Of a traditional social group", "Relating to kinship groups", "Ethnic, sectional" and "Characterised by strong group loyalty".)