Ancient tribe back home to the east of rocks (5)
I believe the answer is:
iceni
'ancient tribe' is the definition.
(I've seen this before)
'back home to the east of rocks' is the wordplay.
'back' says the letters should be written in reverse.
'home' becomes 'in' ('he's home' can mean 'he's in').
'to the east of' is a charade indicator (letters next to each other) (some letters go to the right of others).
'rocks' becomes 'ice' (both can informally mean diamonds).
'in' in reverse letter order is 'ni'.
'ni' put after 'ice' is 'ICENI'.
(Other definitions for iceni that I've seen before include "Old tribe" , "Rebellious tribe" , "East Anglian tribe of Britons" , "Ancient Brits" , "Ancient British tribe" .)