Keeps turning nastier (7)

I believe the answer is:

retains

'keeps' is the definition.
('retain' can be a synonym of 'keep')

'turning nastier' is the wordplay.
'turning' is an anagram indicator.
'nastier' with letters rearranged gives 'RETAINS'.

(Another definition for retains that I've seen is " Holds back, keeps".)

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