Seriously stumbling experimentally; not terribly plain (9)
I believe the answer is:
'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" .)