Fierce critic got a racist sacked (10)

I believe the answer is:

castigator

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'fierce critic' is the definition.
The definition suggests a singular noun which matches the answer.

'got a racist sacked' is the wordplay.
'sacked' indicates anagramming the letters (sack can mean to plunder or destroy).
'got'+'a'+'racist'='gotaracist'
'gotaracist' with letters rearranged gives 'CASTIGATOR'.

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