Artist’s Sleep Activity/Mark Twain’s Head (9)

I believe the answer is:

rembrandt

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'artist's' is the definition.
(Rembrandt is an example)

'sleep activitymark twain's head' is the wordplay.
'sleep' becomes 'rem' (abbreviation for Rapid Eye Movement).
'activitymark' becomes 'brand' (I can't explain this - if you can you should give a lot more credence to this answer).
'head' says to take the initial letters.
The initial letter of 'twains' is 't'.
'rem'+'brand'+'t'='REMBRANDT'

Can you help me to learn more?

(Other definitions for rembrandt that I've seen before include "Influential Dutch artist" , "Dutch painter, d. 1669" , "Dutch painter, 1606-69" , "'Night Watch' painter" , "One notably exhibited" .)

I've seen this clue in The Guardian.

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