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Most fretful sorceress, one participating in experiment (10)

Ross

I believe the answer is:

twitchiest

'most fretful' is the definition.
(both can mean anxious or nervous)

'sorceress one participating in experiment' is the wordplay.
'sorceress' becomes 'witch' (I've seen this before).
'one' becomes 'i' (roman numeral).
'participating in' is an insertion indicator.
'experiment' becomes 'test' (test is a kind of experiment).
'witch'+'i'='witchi'
'witchi' going inside 'test' is 'TWITCHIEST'.

(Another definition for twitchiest that I've seen is " most ill at ease".)

I've seen this clue in The Times.
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