Crossword Genius

Seriously stumbling experimentally; not terribly plain (9)


I believe the answer is:


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'plain' is the definition.
Both the answer and definition are adverbs. Maybe there's an association between them I don't understand?

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for extremely that I've seen before include "To a high degree or extent" , "remarkably" .)

I've seen this clue in The Telegraph.
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