Decorate 50 per cent of nice old Brits (5)

I believe the answer is:

iceni

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'old brits' is the definition.
The definition suggests a plural noun which matches the answer.

'decorate 50 per cent of nice' is the wordplay.
'decorate' becomes 'ice' (to ice a cake is to decorate it).
'50 per cent of' suggests halving the letters.
'nice' halved is 'ni'.
'ice'+'ni'='ICENI'

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(Other definitions for iceni that I've seen before include "Ancient Brits" , "Queen Boudicca's people" , "Old people" , "Boudica's tribe" , "Ancient tribesmen" .)

I've seen this clue in The Times.

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