Group of related families (5)
I believe the answer is:
tribe
'group of related families' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tribe that I've seen before include "A set or class of people, may be primitive" , "Breed" , "Close-knit social group" , "Division of, eg, North American natives" , "Ethnic group" .)