Group of families (4)
I believe the answer is:
clan
'group of families' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for clan that I've seen before include "Tribal group with common ancestor" , "Extended family as in Scotland" , "People related by blood and marriage" , "Group of families with common ancestor" , "Large family forming close group, big in Scotland" .)