Ancient British tribe or nice new one (5)
I believe the answer is:
iceni
'ancient british tribe' is the definition.
(I've seen this before)
'nice new one' is the wordplay.
'new' indicates an anagram.
'one' becomes 'i' (Roman numeral).
'nice' is an anagram of 'icen'.
'icen'+'i'='ICENI'
'or' is the link.
(Other definitions for iceni that I've seen before include "Queen Boudicca's people" , "people rebelling against Rome" , "Ancient British tribe" , "Old people" , "Ancient Brits" .)