Yes, wet skin upset chief testifier (3,7)

I believe the answer is:

key witness

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'chief testifier' is the definition.
The definition suggests a singular noun which matches the answer.

'yes wet skin upset' is the wordplay.
'upset' indicates anagramming the letters.
'yes'+'wet'+'skin'='yeswetskin'
'yeswetskin' is an anagram of 'KEY WITNESS'.

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I've seen this clue in the Sydney Morning Herald.

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