Primary social groups (8)
I believe the answer is:
families
'primary social groups' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for families that I've seen before include "Groups related to one another by blood or marriage" , "Kinship groups" , "Parents and their children - lots of them" , "Primary social groups" , "File Siam wrongly under social units" .)