Mistakenly state purity in case of parentage (9,4)

I believe the answer is:

paternity suit

'case of parentage' is the definition.
(legal case concerning parentage)

'mistakenly state purity in' is the wordplay.
'mistakenly' is an anagram indicator.
'state'+'purity'+'in'='statepurityin'
'statepurityin' is an anagram of 'PATERNITY SUIT'.

(Another definition for paternity suit that I've seen is " A who's-the-daddy action".)

I've seen this clue in The Independent.

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