Invalid felt sicker, having come out in most spots (10)

I believe the answer is:

freckliest

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'come out in most spots' is the definition.
I can't judge whether this definition defines the answer.

'invalid felt sicker' is the wordplay.
'invalid' indicates an anagram.
'felt'+'sicker'='feltsicker'
'feltsicker' is an anagram of 'FRECKLIEST'.

'having' acts as a link.

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