Sitter got out, most dirty (9)

I believe the answer is:

grottiest

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'most dirty' is the definition.
Both the definition and answer are superlative adjectives. Perhaps they are linked in a way I don't understand?

'sitter got out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'sitter'+'got'='sittergot'
'sittergot' is an anagram of 'GROTTIEST'.

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I've seen this clue in The Daily Express.

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