Old Brits located trendy centre in France here (5)
I believe the answer is:
iceni
'old brits located' is the definition.
I can't tell whether this definition defines the answer.
'trendy centre in france here' is the wordplay.
'centre' indicates the central letters.
'in' means one lot of letters goes inside another.
'france here' becomes 'ici' (I can't explain this - if you can you should believe this answer much more).
The central letters of 'trendy' are 'en'.
'en' placed inside 'ici' is 'ICENI'.
Can you help me to learn more?
(Other definitions for iceni that I've seen before include "Ancient British tribe" , "British people once" , "Boudicca's tribe" , "Ancient tribesmen" , "Old tribe" .)