Geeky extremists and most monkish followers are good at somersaults (9)
I believe the answer is:
gymnastic
'are good at somersaults' is the definition.
The answer and definition are not the same part of speech. However, adjectives and past participle verbs can sometimes mean the same thing.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for gymnastic that I've seen before include "Very agile" , "Involving exercises" .)