A twister does so suddenly, perhaps painfully (6)
I believe the answer is:
tweaks
'a twister does' is the definition.
Although both the answer and definition are verbs in their -s form, I cannot see how they can define each other.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for tweaks that I've seen before include "Pinches and twists, perhaps nose" , "Makes little adjustments" , "Improves a little" , "Pinches and twists, maybe a nose" , "Pinches and twists sharply" .)