Top artist involved with Sir — artist being one of these? (12)

I believe the answer is:

portraitists

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'artist being one of these?' is the definition.
The definition suggests an adverb but the answer is not.

'top artist involved with sir' is the wordplay.
'involved with' is an anagram indicator (involved can mean confusing or complex).
'top'+'artist'='topartist'
'topartist' mixed up with 'sir' gives 'PORTRAITISTS'.

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I've seen this clue in The Times.

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