Rebellious Brits get rid of National Insurance (5)
I believe the answer is:
iceni
'rebellious brits' is the definition.
(the Iceni were an Iron Age tribe of Britain)
'rid of national insurance' is the wordplay.
'rid of' becomes 'ice' (I am not sure about this - if you are sure you should believe this answer much more).
'national insurance' becomes 'NI' (UK tax).
'ice'+'ni'='ICENI'
'get' acts as a link.
Can you help me to learn more?
(Other definitions for iceni that I've seen before include "East Anglian tribe of Britons" , "Ancient British tribe" , "Rebellious tribe" , "Boudica's tribe" , "Boadicea's tribe" .)