Family group back in eternal city (4)
I believe the answer is:
clan
'family group' is the definition.
(I've seen this before)
'back in eternal city' is the wordplay.
'back' says the letters should be written backwards.
'in' indicates the answer is hidden within the clue.
'nalc' is hidden in the letters of 'eternal city'.
'nalc' written backwards gives 'CLAN'.
(Other definitions for clan that I've seen before include "Group of related families" , "Group of families with common ancestor" , "One from Benghazi" , "Tribal family" , "Kin group" .)